Water leaks in my XJS celebration 4.0

Hi All,

I'm the proud owner of a 95 XJS celebration but am having some problems with water ingress.

I park the car facing upwards on a sloping driveway and recently found that there was water in both foot wells pooled in front of the rear seat, the front carpets seem dry. When I took it to the garage they tried to find the leak using a hose but couldn't find anything.