Has anyone else come across this problem - I've got an original iPad 3G & wifi that I've had since launch. I've upgraded to iOS 5 and now it's telling me I have no sim although it's got exactly the same sim in it that it always had

Have you tried powering the device off and back on again? Removing the SIM, and putting it back in again, etc? See if that fixes things at all.

Tried all that and doing a complete wipe & restore from back up but no joy may have to go to the Apple Store