Z3 Roof Leaking

[BertBert]

The roof on Mrs Bert's elderly Z3 is leaking. It comes in from the front rhs corner where the windscreen pillar, roof and door meet. There is not an obvious fault (ie I can't see what's going on!).

Is this a common problem and any thoughts on who to take it to for a fix? BMW Dealer or car-roof-person?

Thanks
Bert