Thanks for the suggestions, but unlocked Galaxy Ace > £100, unlocked Apollo > £90 (or cheaper but "out of stock"). For me that's the price of a main phone, I suppose I am cheap in that regard. But I'm not excited by mobile phones, I just want the same thing as worked 5 years ago (or do I mean 10?)
I'm refining the requirements as I go, it occurs to me that I was implicitly thinking of a very non-smart phone, so I was taking it for granted that I'd get long battery life. But actually that's a wish too.
I might just give up on the USB charger port, and then I'd get small, light, very cheap, and 5-day charge life.