Would you believe mine doesn't leak?
The biggest leak was from the lower corners of the hardtop to windscreen seals. One is a big rubber jobby and the other is a foam strip. The water was getting in under the foam one and then round the end of the rubber one. A roll of nice thick closed cell sealing strip from CPC in Preston and a new rubber seal bonded on (see below) fixed that one.
Next was the vent flaps, which didn't compress the seal at the top even when shut. Remove them, ease the hinge away from the flap slightly and put them back on.
Next was the joint half way back along the roof, where the panels overlap. Step one - after a week of hot weather, apply Captain Tolley's Creeping Crack Cure until it stops disappearing. Then apply copious amounts of Frosts' Seam Sealant to every seam, inside and out, including inside the gutters and round the rear hardtop vents. (both products from Frosts Automotive Restoration). Have to re-paint the roof soon...
Paranoia (and a suggestion on AN Other discussion group) suggested filling the longitudinal ridges on the roof (if you have them) with builder's foam through holes drilled upwards from underneath. I've done this, but can't tell if it's made any difference or not.
Lastly was the ill-fitting roof to rear sides seal, which are ridiculously expensive. I eased the joint apart after undoing the 65000 bolts and added a run of self-amalgamating tape, then did it up again.
Bingo. Dry. Still dry.