Ok. I've done some testing. It turns out that the rubber surround (around the glass of the roof) is where the water is getting in. I reckon the drain holes are fine. So, any of you guys got any ideas on how much it would cost to replace the rubber piece, and how difficult it a job this is, and most importantly whether anyone knows of a garaged in Southwest London that could possibly do the job without charging me an arm and a leg! All comments gratefully received!