I have recently had problems on my TD5 and replaced both roof light seals and guess what - not a bit of difference, I then decided to mastic round the curved roof window glasses, you've guessed it, no change. Bearing in mind I had already cleared the drainage tubes with a nylon "fish" - the answer in the end was the joint between the roof and the upper sunroof plastic frame, I waited for a nice not day and used about a dozen or so flat blade screwdrivers to ease the lip up, at the same time drying any moisture with kitchen roll then gradually work your way round the frame with a mastic gun ensuring that you get a good coating between the bodywork and the frame. After application clean up using plenty of rags and white spirit. Its a messy job but has cured my leaking problem - just make sure that you use a mastic that does'nt dry out. I used black gutter seal from B&Q