Phoned the helpdesk, they did all the usual 'clear your cookie cache' stuff - their final advice was to disable my 'autofill', and if I couldn't remember my password, request a new one online.
Did this nearly an hour ago and the new password hasn't arrived yet.
I did explain that I can log in OK, so I don't understand why I have to tinker around with the login, passwords etc - it's once I've logged in, I can't get to the actual tax return page. I'm convinced the problem's at their end not mine.
As if tax returns weren't sh*tty enough without this palaver ...