ProDave

Henley blocks and your bank of switch fuses need to be withing 3 metres of the suppliers meter. Simple.

Often the first sign of trouble?

Leave them alone. Some future electrician will thank you when he can thread a cable through the existing hole rather than drill a new one.

A "Bulb" is something you plant in earth and it grows into a plant.

Don't fit UFH in this situation. Your overlay boards will not have anything like enough insulation to sit directly on an uninsulated floor. If you do proceed expect a lot of your heat to go down and very poor performance.

Put all your "plant" stuff along the back wall of the garage, and I bet you will still fit 2 cars in there.

What restricts you to 40mm? What is the bottom of that 40mm? 25mm battens UFH pipes between pug mix and 20mm engineered wooden floor is what we have, only 5mm "too thick" for you. What insulation and floor build up below your 40mm starting point?

And unless the run to the sub boards is long, once fused down 16mm is plenty.

I would go straight from meter to henley blocks then feed each SWA with a switch fuse.

Henley blocks don't provide isolation and over current protection of the submains, so your electrician is correct.

Here in the Eastern Highlands, -10 is not uncommon so I sized my ASHP for that. Heat loss at -10 and +20 inside is just over 2kW so at -15 it would still be under 3kW so a 5kW ASHP does the job nicely. I think -18 is the coldest night so far but that is not a sustained cold. So your heat losses are coming out about twice mine for a similar sized house. Defrosting is not usually an issue well below 0, there simply is no moisture in the cold air. Defrosting is mostly an issue around and a few degrees above 0.

There is no hard or fast rule. Try rigging it up on a bench before comitting just to ensure there are no problems. By the way I never use low voltage downlights, that is just one more thing to fail. I much prefer GU10 mains downlight with LED lamps in them. I walk a mile from any that do not have replaceable lamps, because you can almost guarantee in a few years when a sealed one fails you can't buy identical replacements.

My money is two 2 port valves, one for radiators and one for UFH, and the feedback microswitch in the UFH one has failed. Are there any 2 port motorised valves anywhere? Of course this is basic stuff and it is a poor show that plumbers and electricians have failed to find the fault. I hope you have not paid them?

Ignoring the tree you might have a bigger problem. Is this foul drainage with a treatment plant or rainwater? Check your local building regs. Here in Scotland a drainage field must be 10 metres away from a road. That is what scuppered us, but drainage to the burn came to our rescue. What is behind you? A field? Can you get permission to drain there?

Also poor design compared to UK 13A plug. Where are you to have this in use?