Well speaking as a British contracts lawyer, who speciliases in consumer purchase contracts, I'd say you definately do (providing your in England).
I'd also add (as I've been saying before) your entitled to charge cryptic for each time you cannot log in to compensate you for your "loss of enjoyment".
Normally the phrase is "pics or it didn't happen," but this time I'm going with "credentials or you're not."
As has already been said, a law suit wouldn't be worth it. Since the account is for the life span of the game, it wouldn't even be worth a class action suit to get a pro-rated refund. Suppose the problem is fixed tomorrow. What's one day out of an unknown lifetime of the game? Might as well be a prorated value of nothing.