"even t mobile say they are baffled"
Talk to their call centre and suggest that they credit your account with £10. The go to any T-Mobile (or EE) shop and ask for a new SIM. You'll be charged £10 for it.
It may do the trick - you'll retain your phone number, but you'll need to back up the old SIM before you buy the new one, so you still have your contacts list etc. The old SIM will stop working within minutes of you leaving the shop, and the new one will work immediately.
It's worth a try, as it won't cost you anything.