dpmiller

have you now checked the motorised valves and their switches for function? Might be worth looking next at the tank stat too?

Looks lovely, and is very much what we aspire to, some time next year hopefully.

stats don't matter as the wiring box interfaces the two ends...

hang on a mo... There's a cavity between timber and block that needs closing with a firestop too don't forget, and it'd expedient to mount the window to that rather than the blockwork.

Odd that the Scottish regs are still so woolly whilst in NI they were very specific about MVHR even in 2012- which is the latest regs... just in case it's ony use: http://www.buildingcontrol-ni.com/assets/pdf/TechnicalBookletK2012.pdf

Is the DIBt essential? It was alleged to me that the only real difference was a self-closing door and loads of documentation...

I vote for sh*t happens, and there's a dodgy capacitor or backup cell somewhere which has been hiding, waiting for a power fail.

car wiper refill with the rearmost rib sliced off?

sorry... 200mm

what about the other way- I've 110mm rainwater pipes to tee into 20mm perf twinwall. Holesaw, or just let them lie in the drain stone beside the pipe?

^this. I had some holes designed into our steels that match in with the pozijoist webs. Very handy.

you don't "keep the heat in", it takes flow to make heat in the rad. You're looking for a temperature differential across the rad so it (say) flow is 55c, the return should stabilise at (say) 45c. Less flow= greater differential. The pump can only flow a certain amount so if some rads have too much flow others will lose out- that is, have too much of a differential.

means the tank is unpressurised, it vents through the tap. Our Brita filtertap is the same...

Dunno just how tight funds are, but what's wrong with precast?

but as long as you don't mind the Fgas side of things there's a Split here: https://www.ebay.co.uk/itm/air-source-heat-pump/223736171110