There have been a number of threads on this topic.
No definitive answer, but the suspicion points to the screws used to hold the rear bonnet hinge to the woodwork. It is supposed that a flaw in the timber is opened by the screws and under the pressure of rain they slowly leak.
A couple of wood repair liquids have been mentioned, I'm sorry I can't recall which one was considered the best.
Million dod a lot of research into the causes, his Roadster suffered from the problem, but is now cured. Drop him a pm.
