in theory its valid from the moment your transaction is processed, but technically your not a member until your card arrives and you sign it, which takes 2-3 working days, plus 2-3 days postage. so you wont be able to use it for hut discounts, and if anything happens you will be in a bit of a grey area so make sure you take any proof of payment and confirmation emails etc with you just in case.
In reply to peat:
As I understand it they won't cover you unless you have the card signed before you go, I had to query this and this was the response I got from them.
Very good policy though.
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