Help! Water leak issue

I picked up my Rover 75 V6 Connoisseur SE yesterday. It is in excellent condition all round. However once I got it home I discovered the rear nearside carpet is sodden. I see from other threads that this is common problem, although originating from various problems. I have traced the source as coming down through under the rear parcel shelf, dripping down into the spare wheel recess and then under the rear seat. I think therefore it must be coming in through the rear windscreen seal. Has anyone else encountered this and am I thinking along the right lines please? I will of course take it back to the dealer in the next day or two for repair, but would like to figure it out myself first.

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