Thanks again. I've had another look. I've not got another piece of tube to test your theory. But I've opened up the headlining a bit more and pulled out the light above the sunshade and when I fed the wire through the drain again it came out through the hole where the light fitted. So it's pretty clear where the problem lies.

Hopefully it's not a case of a new sunroof cassette. I think it's just refitting the drain tubes. I am going to have a scan on the net to see if anyone's removed the headlining.

My cars well out of warranty territory but I may visit the dealer and see what they say. If it is such a common problem then maybe there's something they will do...