Ok, apologies if this message is repeated - I lost my i/n connection during an earlier attempt to make this post.
Firstly thanks to all who responded, publically and privately, making the effort to post pics.
This is where I'm at. I'd bought an old bike, a Raleigh Royal, which was bit of a 'barn find', a 1989 model but clearly hardly used. I stripped it down and was surprised that the middle chainring appeared quite worn. I now realise it probably isn't worn at all - it's just the way the teeth are cut as per minifreak's post above. In fact the chainring I have is actually in an even better condition than the one one above.
So, I'm sorry (or should that be happy?) to say that I don't actually need another ring.
Apologies to all but thanks for your responses.