Site is Foundation, and problem page utilises several ‘Reveal Lightbox’ stacks from Foundation-Dynamic Layout set. Some of the Reveal lightbox stacks have jpg images and a couple use De Fligra’s NiceFrame stack to iframe remote content.
When I had 4 ‘Reveal Lightbox’ stacks all was well. Since adding another two, the page has stopped scrolling. However, simply removing the last two doesn’t make it scroll again - it all seems to be more random.
I have checked on Safari and Chrome - the page just doesn’t want to scroll down any more.
Problem Solved - there is an option in Nice Frame to disable browser scrollbars and one of the Nice Frame stacks had this on. Moral of the story - don’t work late at night when half asleep