Found the problem.
The channel that drains from the windshield terminates directly above the metal sleeve that the drain tube passes through, which in turn is directly above the wiring harness that enters the car through the lower grommeted aperture. Water flows from the channel, on to the pass-through pipe and via gravity to the wiring harness eventually eroding the grommet seal allowing rainwater into the footwell.
This is a design fault and will eventually cause leaks in all cars - in my case I hadn’t noticed a problem until this fall when I lost my covered parking and moved the car on to the street.
I’ve tried re-routing the stream from the channel using a length of hose …this may work but if not I also cleaned out the grommet slot and applied sealant to the join.
Here’s a pic from another car that leaked in this way which eventually ate away the grommet aperture. There is no way that this area gets subjected to rust other than from water running down the harness from the ill-placed drain channel.
Really stupid design, Jaguar!! 
