There is a lot of misinformation in this thread.

Firstly, insulation of the house has nothing to do with HP's, it affects all heating sources, so I really dont know why HPs are singled out as 'they dont work on old properties', as, clearly they do! Of course insulation helps, and more insulation is the biggest thing to do to save energy.

Ground vs Air. I used to be an advocate of Ground source, but no longer. I had a call out to one, it had frozen where the pipes came out of the ground, and the frozen ground went around 3 feet down. Research later showed that this was a known problem if the pipes are too close to each other in a vertical excavation, and also happens if they are too close to the surface in a horizontal format. The UKs typical air temperature is above 5 deg. C for 95% of the year, the ground will drop to to 5 deg before christmas on a horizontal excavation, and a bit later on a vertical piled array. They wont benefit from a daytime at 10 degrees, which is quite common, even in January. Vertical arrays are far better, but the install cost is beyond most normal people, and unlikely to ever pay back the install cost, even the excavations for a horizontal array at 2 metres deep may not be cost effective for 20+ years.

Have a back up. Well, you've answered that, a gas boiler or immersion wont work if there is a power cut. A gas boiler will only consume around 250 watts power, so a generator can easily supply the power to make that run. If you have a heat loss of 3kW, then a larger generator, of 2kW or so would run the HP succesfully. Being as you have a log burner, with a back boiler,you are sorted anyway. Yes, a HP, and solar/solid fuel can be combined succesfully.

Frost. Yes, they do frost up in humid conditions below 5 deg C. The defrost cycle will take some of the water from either the central heating, or hot water cylinder supply loop, run it in reverse (not actually in reverse, a valve opens inside the HP), run that for 2 or 3 minutes, and the defrost is finished. Of course, the COP does drop because of that, but, the HP should, if designed and fitted correctly, achieve a COP of 3.5 or higher. I was on a course last week, the Owner of the Training Centre is a bit of a researcher into HPs, and he regularly gets a COP of 6, all year round. He said on the day before, it was almost 9.

Cost. Install cost can be almost free now with Octopus et al falling over themselves to fit them. However there is a downside to the cheap install cost. BG guarantee that their HPs will keep your house warm, but they dont say at what cost. From some inquiries, they set them to run at 50 deg.C which is not at all efficient. Octopus are similar in that they do a 'one size fits all' approach. If its cold, they just turn up the flow temperature, which gets the house warm, but adds energy costs.

If you do want one, then it shoud be designed correctly, with a good heat loss survey done before anything else. From that, it can be decided if the current system needs upgrading. At a 3kW heat loss for the whole house (getting toward passive levels?), then you will probably be ok for the piping through the property if they are 15mm+. Radiators/emitters may have to be changed.UFH pipes are generally ok, as they usually run at a lower temperature anyway (27-30 Deg C?) The flow temperature will certainly be lower with a HP compared to a gas boiler. From the flow and return temperature is the Delta T, temperature difference. On a gas boiler it'll be 20-40 deg. on a HP it'll be 5 to 7 if designed correctly. This is what causes the need for larger emitters, not the flow temperature, basically, with a smaller DT, there is less heat given out on each revoltuion of the heated water, so, to make it up, a larger emitter is required.

Running costs. Without good research, it can be hard to justify a HP against a gas boiler. Install cost is the big problem, as said, it can be fitted cheaply, with the 7.5k grant, can be almost free, but I wouldnt trust the Company who fitted such a HP. There are better Companies out there who do a far more detailed design and installation, and give proper after service (at the least, i would expect a revisit after a week, then a month, and then 6 months on to ensure all of the settings are optimised, being as you are technically minded, that could be something you do yourself). You would not get that kind of service from Octopus, or, indeed, any of the larger Companies who have jumped onto the £7.5k grant bandwagon.

If you get a SCOP (years average) of 3.5, then the running cost will compete with gas. If designed properly, I think you could expect a SCOP of over 4. Get below 3.5, and gas will be cheaper. But, who is saying gas will not increase in price? It certainly wont decrease by much, whereas electricity could come down in price slightly, some suppliers are actually giving free power at certain times now as wind and solar need to be used up in their generating peaks.

Whether to get one. The main advantage is their lack of emissions at source. Yes, there is still some coal, and a lot of gas generation, but that percentage is slowly falling, so you can say, without doubt, that you are running a greener form of heating, which will only get greener in future as renewables increase their production.

Cost, install is certainly more than a gas or oil boiler. Running costs should be comparable to, or better than, a gas boiler. If it isnt, then the design and / or install has been done badly.Ongoing costs should be similar. They should be serviced every year (Grant do a 7 year warranty on their HPs if serviced to their spec), the service cost is typically £150. If you have a pressurised cylinder, that should also be checked every year, so combine the 2. Servicing costs are a little more than gas boilers, but, most gas services do not do a correct service, look into your service manual for any recent boiler, and there is a long list of items needing to be removed and cleaned etc, whereas most Gas Servicers clean out the combustion chamber, do a combustion and gas leak test and say it has been serviced. BG were notorious for this lack of actual servicing.

It's a difficult choice, I'm getting a ASHP myself after looking into it a lot for my own house. Over the last few years I've done a lot of Courses about heating, and manufacturers courses on their own HPs. I wouldnt recommend one make over the other, but I'm going for a Daikin 4kW unit. Nibe was a close second, and has a better set of controls, Grant was in the frame too, but the training and back up from Daikin won the day, and the price was a bonus too, in being very slightly cheaper than the Grant, the Nibe was a lot more expensive, but I think it is a better overall unit, with better controls, but the extra cost did not justify that over the Daikin. Vaillant have been the market leaders for the last 18 months or so, but many of he other makers have caught up with them. Its a minefield, if you were buying a gas boiler, it is far easier to make a choice, as most of them are very similar, HPs are harder, as they all have their own proprietary controls, and the controls and setting them up are a big part of making them easy to use. If you are still looking next June, then get yourself along to the Installer Show at the NEC Brum, as all the Makers have their products on show in one big hall.

The problem with this is the limited power of an ASHP. We would need about 8 of them. Where would we put them, and what about the neighbours?

How on earth can you not connect insulation (ie heat loss) in a building to how much demand is placed on a heat source?

If a house is not well insulated that heat source (whatever it may be) will have to generate more heat to counter the heat loss.

The simple fact is that heat pumps are nowhere near as effective as gas boilers.

Also you can only insulate so much, you get to a point where you get diminishing returns and more insulation becomes pointless.

For most of us there is no valid reason to get a heat pump, asides from pressure/policy from the government based on senseless green washing.  I'll be sticking with my gas boiler as long as possible.  It is far cheaper to run than an electric appliance, with limited effectiveness.

No, I stated that it is quite common for people to say heat pumps don't work in poorly insualted properties. Well, actually, they do work, and insulation affects all heat sources, not just HPs.

Of course insualtion is a good thing, and should be better in all houses, my point was the often quoted saying that 'HPs dont work in poorly insualted houses' is totally wrong.

HPs are far more efficient than gas boilers. The best quoted performance figure for a gas boiler is around 95%, and that is under test conditions, the vast majority of installed gas boilers do not meet that figure. A very poorly installed and designed heat pump will achieve a COP of 2 or higher, the better designed one will easily achieve a figure of 4, - put 1 unit in, you get out 4 units of heat.

At a COP of 4, the HP will be cheaper to run that a gas boiler. Add in some of the better electric supplier deals, and you have a cheaper source of heating and hot water than any gas boiler can get to.

I'm not sure where this hate of HPs comes from, they are the way forward, figures are there to show they are more effficient and  should be cheaper to run than gas boilers, but people keep putting out these myths that they dont work.

Are you talking lab or real world?

Heat pumps will not work as well in poorly insulated houses because the heat load is higher, same with any other heat source, they will have to work harder and with any heat source it may be beyond their capability.

Heat source pumps rely on temperature difference to operate effectively, when that difference is great they are not as effective.

Couple that with poor insulation and they may struggle.

That's before we even talk about the cost.

I'll be sticking with gas thanks very much and I would recommend the OP to do the same. 

Exactly, you need far more of them especially in a larger house and this would multiply the cost to install and run.

Stick with gas, don't change your boiler unless it is broken (or it is apparent that you cannot buy a replacement in the next few years).

Err, the first thing you do it to commission an heat loss survey. You then size the heat source to provide the heat requirement. This is the same for any heat source. What has happened over the last 40 years is that those skills are not done, and any boiler is thrown in, and, it is usually oversized. This is good for the heat, as it'll output, probably, double the required heat output, but, it is inefficient.

HPs are designed. Not just thrown in, and that is why there are some poorly designed and installed heat pumps, the people fitting and commisioning them do not have the skills to do it properly.

You can carry on using your gas boiler, no one is forcing you to change it, but, if you take your blinkers off, you will see that HPs are a better source of heat for the majority of domestic houses. There will  be some that cannot have them, just as there are houses that would not be suitable for other types of heat source.

Err, the first thing you do it to commission an heat loss survey. You then size the heat source to provide the heat requirement. This is the same for any heat source. What has happened over the last 40 years is that those skills are not done, and any boiler is thrown in, and, it is usually oversized. This is good for the heat, as it'll output, probably, double the required heat output, but, it is inefficient.

HPs are designed. Not just thrown in, and that is why there are some poorly designed and installed heat pumps, the people fitting and commisioning them do not have the skills to do it properly.

You can carry on using your gas boiler, no one is forcing you to change it, but, if you take your blinkers off, you will see that HPs are a better source of heat for the majority of domestic houses. There will  be some that cannot have them, just as there are houses that would not be suitable for other types of heat source.