admin okay thanks, I've no immediate need to plug in during the day/evening, and am reluctant to do it to be honest! It's a bit worrying that an apparently stable system can suddenly go wrong like this!
I will take careful notes of timings as you suggest next time I do it.
Givenergy have still not got back to me (I believe it's still only Installers that can phone in?), and its not exactly an ugent issue now - but I am disappointed in the customer 'support' that GE now seem to be giving, as it took them ages to respond to my last contact as well. I frankly wish I hadn't got their kit!