Fixed!
The .co.uk account does indeed provide free pop access, you just have to sign up to a bit of occasional spam first (Yahoo Directions, I think it's called). Compared to what I already get, I doubt I will notice... From what yahoo support told me, I think the web site was supposd to point me in this direction automatically, but it didn't. Anyway, now i can read the mail on the browser (and delete spam without downloading it, which is why that bit really matters) and also download it to the mail program for off-line use. The SMTP authentication still doesn't seem to work but that doesn't matter to me.
I guess this will work for anyone - I only signed up for the account after moving to Japan. it just has to be the .co.uk site not the .com one. Maybe .co.jp works similarly.
James