Water leak

[Grandad Steve]

I recently found the drivers side footwells, (front and rear) flooded. This has only happened recently after a humdinger of a rainstorm. I'm pretty sure it leaked forward of the rear seat, maybe around the front bulkhead or door area. I've dried it all out, and looked for traces of watermarks, but non are evident. I'm now in the process of getting a new sensor to fit. The plenum drains are clear, with no sign of water ingress. I've also replaced the drivers door seal, as it was showing signs of wear. Anyone any ideas please?
Cheers