FWIW I've found the leak. There is a hole beneath the kick panel on both sides, at floor level behind the carpet, and partly hidden by the electrical conduit box. This is an oval about
50 cm x 30 cm, a little behind the drain hole in the bottom rail, and allows some wiring inside from the bottom rail . This hole is much larger than the wiring and has no gasket or cover. The local dealer parts counter is sympathetic, but they have no such part. The inside of the rail is coated with gritty grime that has been blocking the small holes in the drain hole grommet, causing water to back up and flood the floor. This grommet has been cleaned several times but I suspect it has been blocked up again as soon as I drove long enough through a hard rain.

I would be interested to hear from anyone who knows what should be filling this hole (surely there should not be an open hole in that place).