Thanks for your reply.

The *only* thing that has been done on our car is the speaker in the rear passenger side never worked, so at the last service it was fixed - connectors weren't pushed on.

I wonder if the dealer disturbed the vapour barrier (or whatever its called) and this has led to the water ingress?

As you say it is the actual carpet thats damp, not the mats. Indeed, the mats have a rubber backing so I know it isn't soaking through.