Missed my first credit card payment, have i now messed up my credit rating?

It might be that you didn't get a late payment marker at all. Rather than directly report a missed payment, CCs report the status of the account monthly. If at the time of report you are up-to-date, you won't have anything negative reported.

As for losing promo offers, the "gotcha" has always been part of the business model. CCs make the offer because they know that a certain proportion of cardholders will end up paying standard interest on a hefty balance, either because they run off the end of the deal, or they miss payments.

That said, to remove a promo deal because of a single missed payment is legally dubious (whatever the T+Cs say) in my view. It is a disguised penalty. CCs have become softer on this - at least one provider doesn't remove it at all, others let you off for a first/second "offence" (whether or not you request it), and the backdating of interest to the start of the statement period for which you missed the payment appears to be stopping. The last thing they want is a regulatory invention. In a few years' time I'd bet that the gotchas will stop - and therefore BT deals won't be so enticing.