Just to throw a spanner in the works.

I contemplated buying an Ego inverter that runs off their cordless power tool batteries, then changing the SFCU unit to a plug and socket so if there’s a power cut I can still fire the central heating up.

https://www.ronsmith.co.uk/ego-pad1500e-nexus-escape-150w-power-inverter.html

But my understanding is it won’t work because the flame failure device in my gas boiler completes its electrical circuit through earth and won’t run off that inverter.

So I need to rethink that plan.

We also have a bedroom over an integral single garage, so I looked at the price online of bags of loose fill insulation to put between the floor joists to insulate between the bedroom and unheated garage, that came in around £650 so I had better phone a couple of builders merchants to see if I can get a better price.

However I have just added a blind and a curtain to a window in our porch that’s never been covered, my wife said it’s a bit OTT having both on the same window, but at £35 for the blind plus another £35 for the curtain and £15 for the curtain track I’m sure that the investment of around £85 is a sound one that will soon save that the way things are going with fuel prices.  

Just to throw a spanner in the works.

I contemplated buying an Ego inverter that runs off their cordless power tool batteries, then changing the SFCU unit to a plug and socket so if there’s a power cut I can still fire the central heating up.

https://www.ronsmith.co.uk/ego-pad1500e-nexus-escape-150w-power-inverter.html

But my understanding is it won’t work because the flame failure device in my gas boiler completes its electrical circuit through earth and won’t run off that inverter.

So I need to rethink that plan.

We also have a bedroom over an integral single garage, so I looked at the price online of bags of loose fill insulation to put between the floor joists to insulate between the bedroom and unheated garage, that came in around £650 so I had better phone a couple of builders merchants to see if I can get a better price.

However I have just added a blind and a curtain to a window in our porch that’s never been covered, my wife said it’s a bit OTT having both on the same window, but at £35 for the blind plus another £35 for the curtain and £15 for the curtain track I’m sure that the investment of around £85 is a sound one that will soon save that the way things are going with fuel prices.  

Sparkingchip said:

We also have a bedroom over an integral single garage,

Yes, insulate the floor. I take it you have a wooden floor. You don't say what the m^2 is, but if I guess 20 m^2 then the rock wool needed doesn't come to anything near the price you quoted. 

I have an 80 m^3 room sitting over the cellar. When I had the floor insulated the difference in warmth retention was immediately noticeable, but I didn't measure it.

The most efficient winner overall is insulating between upper floors and roof space. Rule of thumb is 25% savings (that is what more elaborate calculations get, also, +/- a couple per cent). That is almost exactly what I saved in gas kWh's when I had it done.

The easy way is to clear out the roof space and just roll the rock wool out on the floor. That cost me well under €1000 for about 150 m^2 in January 2010 and it saved me that in gas costs that very year.  A couple of years ago, I had the floorboards removed and laid it between the joists, so I could use the roof space for storage again. Probably works just as well, but I don't have the gas bills for enough years yet to confirm that more precisely.