Now I feel like a proper home owner; we had to pay for a heating service at short notice as after a power on and off our boiler didn't come back. The solution was to replace its control board, so /something/ on here is broken. Technician reckoned a relay was the culprit. Don't think I have the energy to try and figure it out, but probably a cheap part that's not too hard to replace. Annoying when it's quicker to just replace the whole thing.