ProDave

This for me is ringing alarm bells that it is NOT the white water expansion vessels at fault. If it were then the tank would be discharging on over pressure and that would be 45 degree hot water from very near the top of the tank. I would still want a plumber I trust to test everything and find out exactly what IS at fault. One thing I did when plumbing my own system is I fitted a pressure gauge on the top of the hot water tank so I can see if it is over pressurising. That is not normally done, but I had one spare. Perhaps when the guys come with their new white expansion vessels you can agree with them, if this does NOT solve the problem they will fit the original ones back and refund that cost? It is sounding to me like "fault finding by substitution"

Are these houses being lived in, or is it as the website suggests a "home exhibition"?

No need for earthing but you will need an enclosed terminal box for the mains connection not just a pair of naked wago's. I would also use an enclosed junction for the DC connections.

Yep, if no services, put the PB straight onto the PP board without a gap. Gives you a little more heardoom.

post the details of the exact thermostat.

Surely a utility room will have a sink unit or something, put the odd ones under there?

You should have counted how many you had before you started that room.......

Tile the WHOLE utility room different. Keep the rest of the white ones as spares for the other rooms.

We have a petrol leaf blower. It can also suck and act like a big vacuum cleaner sucking leaves up into a bag. That function is handy for clearing out gutters and drains, but not of much use for general leaf clearing as it would be soon overwhelmed.

Heating the hot water will heat it hotter than the ASHP so there will be more expansion. But that should not be a problem. You have two expansion vessels for the hot water, I only have one, and my single expansion vessel is still working fine. If you wanted to keep the 2 old ones you could refurbish them with a new bladder in each, or even ask they guy if he would replace the bladders for less than the cost of replacing the whole thing?

Okay progress. We know the hot tank is over pressurising when the water is heated which does indeed usually indicate the expansion vessels(s) have failed. I am a little surprised as there are two of them so would expect them not to be particularly stressed. This is not something to ignore. You have to trust your gut feeling and get the person that quoted to replace them or a different plumber that comes trusted or recommended by a friend. This does not need to be a heat pump plumber as these expansion vessels are the same regardless of whether it is a heat pump, gas or oil boiler, or just plain direct immersion heated. Switch off the solar diverter for now and only heat the hot water occasionally as needed using the heat pump to heat it. A lesson here is be there when the next plumber comes so you can engage with them and when they find a problem like this they can probably show you what they are testing that made them conclude they need replacing. One would hope they had de pressurised and concluded the air bladders are burst and they would be able to show you. EDIT: to add to the above post, if you have a car pressure gauge and a bicycle pump we could talk you though checking the pressure to be absolutely certain if they have failed or just not enough pressure in them.

I very much doubt you will get two 10mm cables into one of the terminals on one of those. Is this a (home made?) storage boiler? the commercial ones will have one high power feed to a control box and typically use a contactor to switch each element.

Before you make the choice, price up all the options. Frametherm is so much less nasty than most wool insulation and it is quite stiff so when pushed in place it should stay there while you get the netting etc in place, many will just fall out. https://knauf.com/en-GB/p/product/frametherm-r-roll-35-ready-cut-26329_4206 Search for best price, I got mine from SIS as they were the cheapest I could find it at the time.

4" will be a lot better than nothing. It will be a miserable job from below you will need full suit and respirator imho. Use something like Frametherm 35, a lot less nasty than most flavours of rockwool style insulation and quite stiff so should push into place and stay there while you get the netting in place.

Where I worked before they built a spiral staircase. The plasterboard was stood against a wall outside and wetted frequently and gradually it curved under it's own weight as it was leaning against a wall. After several days it had got to the approximate bend radius needed so it was brought inside and wetted further and gradually eased into the bend needed. Once to the radius you need you let it dry before you screw it to the wall. That is a job well suited to DIY as it will take time and your own labour is free.