Anyway, reported it, nice phone staff they have. (Kind of the opposite of BT where the phone staff were obstructive & in some cases downright rude, but the engineers were great.) She talked me through a couple of checks that it was not an internal fault (apparently one should switch all the trip switches off, wait a couple of seconds & switch them back on, sometimes that resets it if it's an internal fault), then said they'd get an engineer out - might be 3-4 hours or more, but if I called back in 2 hours then they could give me a better estimate. Couple of hours later, van with flashy lights parked up a bit down the street. Engineers were less good with the customer service side of things - no attempt to contact people (despite me having been told they'd need access to the house - they didn't as it turned out) and when I popped out to ask how long it'd take to fix they seemed surprised I cared & told me it'd be 20 minutes but they were doing the paperwork first! It did come back on by 7:30pm so all was well :) And got a call from the phone staff shortly after to check my power was on & to apologise for the inconvenience.
So, that was yesterday's excitement! It's astonishing how quiet the house is when everything's powered off - we very rarely switch the pcs off, but even when we do obviously the fridge is still going. I sat & read a book for most of the time, by torchlight later as it got too dark to see - we're not that well prepared for the power going off, but we do have torches and some candles. And it turns out that the ignition on the gas fire is mechanical rather than electrical, so we would have had heat had we actually needed it - but it was mild enough that it wasn't necessary. Food would've been the problem, I don't think I've anything in the house that's a decent meal without needing cooked, but we're in a town so takeaway & resturants are an option (and I suspect if the entirety of Ipswich is without power for a significant length of time then there're likely some much bigger problems).
Thankfully all the electronics appear to've survived unharmed, and I'd been just about to go do a couple of chores so I'd saved all the things I'd been working on just 5 minutes before it powered down!