It worked! The check wasn't instantaneous though (it took 5+ mins).
I didn’t see your message until just before we got to the Apple store, but decided to tell a white lie and say that we were still at our old address (we know the people living there anyway) and it all went through with no problems. Happy owners of two new upgrades. We’ll phone up tomorrow and change our address.
thanks for getting back to me, glad to hear it worked for you!