It turns out the issue I was having was to do with the size of my mail box on the server, once that was reduced Mail started working for me again. I've now got it set up to clear mail out of the server mailbox a day after it's been downloaded so hopefully that will keep things running.
Do you have a blackberry or similar that uses the same account (or access the mail account from another machine)? I'm having a similar issue and I think it's my blackberry (or at least the blackberry service) that's locking the account.
I haven't found a solution yet either but will update...
