Thanks everyone who has replied to this thread; very sorry for starting it and not checking back in.

The conversations backup/restore did seem to work. I was good for a few weeks, but now the issue has cropped up again. I can't figure out why it happened. No new software or updates immediately before (IIRC).

I also found this thread, which offers a console-based potential solution. I'll give it a go cos this is driving me up the wall, especially after a reboot when I get 26 'new' SMS from my most recent contact which then appear out of sequence and without my sent messages (thus destroying the conversation).