Forgot I'd started this thread, but I did find a solution to this problem so thought I'd post it here to wrap things up.

Basically, to sort out the login problem on Opera Mobile all you have to do is tick the 'remember me' box that logs you in automatically. This seems to give you the right cookie and keeps you logged in so you can browse the site as normal, complete with gifs working. Came in handy a couple of times recently...