Seems the problem was a coincidence. We swopped my SIM into OH's phone and it worked with no problems... until my credit ran out
(I'm on original PAYG). This failed to trigger my brain and we went off to see our tame Apple expert.
Told her the process we'd gone through and she pointed out the obvious. The replacement SIM worked so there was nothing wrong with the phone and we should call Virgin
We did and a very nice young lady arranged for a replacement card, which arrived withing 48 hours.
So all is good again and harmony is restored