Thanks Nick and Neil,
Very useful information

I took my plip all to bits this evening and managed to flip the transponder module out of the key. As an aside, I was amazed at the range of the RF side of the plip. I dismantled the plip in the garage and the car was a good 30 feet away and still locked as I pressed the button taking it apart
So, If I'm correct, the actual transponder module is the little chip about 8 x 3mm and about 2mm thick. I was hoping I could see an obvious fault but not so

Why they pack up is a mystery as they must be passive. They get knocked perhaps? If I understand correctly Nick, this little module is available as a spare part but needs to be programmed to the car.
The problem is that I do not have the plastic card that should be with the car containing the transponder code. Is this available via a Stealer on production of the car VIN and proof of ownership etc? Any idea of the cost all this will entail?
I'm getting worried only having one key working. If that one fails I'm really up the creek.

I assume it is not easy to unlock the ECU to bypass all of this...