Make sure they are using this address:
The Liverpool Way - Powered by vBulletin www. liverpoolway. co. uk / forum
rather than this address:
The Liverpool Way - Powered by vBulletin forum. liverpoolway . co. uk
because that might be their problem.
It lets them in on the second address, then redirects them to the first address, but they're not logged in on the first address.
Does that sound sensible?
(I think this is tied in with the SEO extension that was added, before that you could use either version and actually be logged in as two different users).
Anyway, make sure they log in on the first one, see if that works!