Thanks all. The problem started about 3 years ago. I took it to a BMW garage who basically said that the proper fix is to replace the sunroof and the pipes, but that was an expensive job and tehy could instead just clear the pipes out and see how we get on. THey did taht and it was fine for 2 years. Then I noticed the boot getting wet again about a year ago and took it back. They said teh same thing, cleared the pipes again but when the problem returned again a few months ago they said taht there was no blockage, so the pipe was probably damaged but repairing it was a pain. Harry - I'll do a bit of investigation because that sounds preferable, but the man at the BMW garage did seem like he wanted to help so I suspect its not as simple as that. But you could be right - it would help explain why so much water makes it into the boot!
I've ordered some of that tape from toolstation (they deliver but don't stock it, unless I'm looking for the wrong thing) so hopefully I can have a go at patching it up over the next day or so. Tonight's job will be to sit in the car while my wife sticks the hose pipe over teh roof to see exactly where it is coming in, so I can see what difference the tape makes!