Ask your provider for a new SIM (or buy one in a shop, they're usually a quid or so). With the network I'm on you can do the porting via txt msg. Works for both payg and contract.
I now have a SIM adapter, my 4S is up and running. I'm expecting a couple of important work calls, so it's a weight off my mind.
Have also in a fit of impetuosity ordered a 6S. Will kick myself next week when the prices drop massively.