Thanks for your replies, folks. In truth, I only own this bike because I have a problem. I can't not buy a vintage road bike when it's going for under a tenner. But I have no room for it, so I'll give it a wipe down and find a new home for it I expect.

Thanks T-Mar - I don't know the actual diameter, but the seat post from my very bog standard 1985 raleigh fits in there, so I'm guessing that's evidence enough of entry level frame quality.