My brit open has the exact same issue, i even bought a "new" roof as mine leaked in the rain, needles to say that the "new" roof also leaked in the rain. as the other guy says its down to the canvas shrinking. you can stop it leaking AND stop alot of the wind noise...
what you will need is a few metres of 10mm hose, a few jubilee clips, T pieces, elbows and some self adhesive cable tie holders..and most importantly a length of shower door rubber!
the trouble with the webasto on the mini is that it doesn't have any kind of drainage, whats that? you think it has? its got guttering and 4 holes, 1 on each corner, the rain must go somewhere.... NOPE, the gutter is bunged off, meaning rain gets under the canvas, sits in the gutter, overflows then pours in the cabin when you go round sharp corners or brake hard. so what you need to do is make your own drainage system (which rover REALY REALY should have done bearing in mind how rust prone the mini is). thats where you utilise the list of parts above to create a drainpipe, which you can make, linking the 4 webasto gutter outlets using the pipe and feed it down the inside of the c pillar and out through one of the holes in the boot floor
why do you need the shower door rubber? to seal the gap at the front of the roof!! the seal that holds up the headlining around that dirty great hole for the webasto, needs to be removed to take the lining down, so you're half way there... cut the shower door seal the width of the hole and use it to hold up the headlining at the front, this will then also seal the gap between the sunroof and the font of the hole.
job as they say is a good'un!!
no leaky roof, lots less wind noise..
i realy should sell my own kits for this little job