Thanks Siamcat - it only happens on that particular thread. I reckon it must be to do with that particular advert.
I thought we'd always had a "next" and a "Previous" button, providing there was more than one page to a thread (i.e. more than 10 replies so that we start a new page)
p.s. This is really Wendy, I was trying out how to register with a different name and yes, it does have to be a differenct e-mail address as Trevor said.