FWIW, my situation is different from Jenbel's. I phoned again, and the very helpful EC lady got on to the airport (LCY) who said that check-in was held while they sorted seat allocations for children etc. I should try again in a couple of hours.

I didn't really understand why OLCI was available but automatically allocated seats couldn't be changed, but she sounded very informed and who am I to doubt. I'll try again.