well, it rather depends how hot the pipes get and how heavily loaded the cables are.

It is not a good idea to take PVC jacketed cable much above 70C, as the PVC starts to age prematurely, becoming brittle and eventually flaking off. Massive overheating - say more than 120C, causes the PVC to flow, and the wires inside to touch, and then things get a bit snap crackle and pop, as per the breakfast cereal.

But, on the other hand, most parts of typicalheating systems don't get much above 50C, as much more than that gives folk nasty burns if they touch the radiators, and wires with an air gap are a lot cooler than any pressed into intimate contact with a pipe. An air gap of a few diameters (pipe or cable, which ever is larger - probably the pipe unless you have microbore heating) is usually enough to get significantly far away in thermal terms, ideally below or to the side, rather than above, from something hot.

Without seeing it, it is unwise to be too prescriptive, but while it is certainly not good practice, unless the cables are already hot due to being fully loaded, and there is nowhere for any air to flow in a gap to convect the heat away a bit, it is not likely to be immediately dangerous.

But any cables that smell of hot plastic or have regions that show noticeable discolouration or blistering from heat should be moved as soon as possible, and where the chance arises and you have the floor up anyway, then slipping a length of the slit and slide foam insulation onto the pipe would be a good idea, if the wire cannot easily be re-routed.

https://www.screwfix.com/p/essentials-economy-pipe-insulation-15mm-x-13mm-x-1m/72357 is an example of the sort of thing - slit long-ways with knife, slip over pipe and tie on with wire or cable ties.

Mike.

Agreed. It's the kind of thing that's far from ideal, but happens rather a lot in the real world and doesn't cause disaster very often. On the positive side most household circuits have quite a lot of margin - e.g. a 1.0mm² lighting cable is probably good for over 6A at 60 degrees C and a 2.5mm² can probably carry 10-13A for ever at 60 - and rather more for shorter periods - whereas the long term average consumption of an non-electrically-heated house is probably below 5A. Loads have typically reduced in recent years (e.g. 5W or 10W LED lamps rather than 50W or 100W halogen or GLS) and appliances generally are more efficient. Likewise gas boilers often run with a flow temperature of 60 degrees or below to keep them in condensing mode (rather than the 70 or 80 degrees that was more usual in the past).

The picture could be rather worse if you had long duration high current loads though, or insist on running the boiler at max flow temperature. Presumably you've not got storage heaters or an immersion along side heating pipes, but EV or a heat pump might be a possibility, or cables serving an electrically heated outbuilding.

the bathroom radiator has a 4mm earth bonding that goes back to the CU

I think someone's rather missed the point there (the idea of supplementary bonding is to tie together exposed- and extraneous-conductive-parts of the location, not reinforce potentials from further afield), but as long as main bonding (if needed) is in place and everything is 30mA RCD protected, it's probably a mute point.

  - Andy.

I agree with Mike and Andy.

Get under the floorboards of any house which has them and you will see a real rat's nest. If pipes and cables are close, move them apart; and if space allows, pop a bit of insulation around the pipes. What I would not like to see is pipes and cables going through the same notch or hole.

This is quite common in many homes.  If you want to address it you can or just leave it and let the RCD do its job.   Modern cables will last for decades touching domestic dwelling heating pipes as it probably only get to about 55 degrees  Celsius
However if you want to address it then get some of the following
15mm pipe insulation
22mm pipe insulation
1 * All Round Band

Now this is where the challenge starts.  Get yourself a hammer and a set of pry bars and a sharpie.  Start to lift floor board, de-nail and mark up for joist positio,n cables and pipes.  Then insulate your pipe work and cable clip/allRound band the cable.  Re-fix floor boards CAREFULLY using screws,  You may need to drill, pilot hole and counter sink.  Floor dependant.   Rinse and Repeat.  It is an easy job but very time consuming and you may also discover other nasties under the floor boards like copious amounts of rubbish and rubble.   CAVEAT Asbestos awarness

On the supplermentary bonding, they have taken them all back to the CU, not sure why. My guess is because the CU is under the batharoom so it was easier?  

I believe my combi heating system runs at around 68°C.

A few years ago, I installed some Screwfix pipe insulation on the bathroom pipes when I had the floor up.

However, insulating the other pipes would be quite tricky without causing significant disruption (and risking my wife's wrath).

Sergio Fernandez said:

Re-fix floor boards CAREFULLY using screws

Not a bad idea! It depends a bit on the finish required and whether it is carpeted or not.

In the instant case, the boards may have been lifted previously, but cutting across an existing board and even worse, coping with tongue and groove is a nightmare. Any tips?

Not all the treasure under the boards is nasty. I have ancient porcelain fuse holders, an empty Woodbine packet and matchbox. They can help with the history of the building.

P.S. make sure that the cat(s) are out when you put the boards back. :-)

coping with tongue and groove is a nightmare. Any tips?

I have two techniques . For modern (softer thinner) solid softwood boards, a Stanley knife (other brands are available) with a nice fresh blade will usually slice through the tongues with a few firm strokes. Failing that a oscillating electric saw (e.g. Fein although mine is a different brand). In the old days you could get a 'floorboard saw' that had teeth around a curve on the top of the blade, which you could use to start a cut from one side, to cut through the tongues, but that could he hard work if the gap was thinner than the saw blade.

  - Andy.

p.s. never try your favourite tenon saw, tiny bits of grit inevitably accumulate in the gap and will ruin the teeth surprisingly quickly.

I suspect that originally, there was a length of green and yellow from the CH to the CU and a further one looped between each set of taps and may be the WC back to the CU.

If the bathroom was refitted, the latter would have been discarded, but if the CH was not touched, that bonding could have been left in place.

My question is whether when a CU is updated to include RCD protection, is there any risk associated with leaving part of the supplementary bonding in place as opposed to either leaving or removing all of it?