Thanks chaps.
Consul now removed, straightforward enough.
I have also removed, cleaned, rustproofed and painted the panel with Hammerite, and I am now sure that water is not coming in from behind that as the plastic sheet behind the panel was intact.
Just to eliminate as many possible causes of leaks I have sealed what I can see on the front bulkhead, I even had the (bright) idea of putting a inspection light in the front, and then shut the bonnet just to see if any light came through into the cabin, but it didn't. I also sprinkled talc on the inside of the bulkhead; as this helped me track the water leak in the rear boot earlier.
It rained heavy during the night and guess what......a small puddle of water in the passenger foot well in front of the seat and by the sill. Its definitely not coming in from the front so it must be coming in from the side, then builds up and sloshes about to the back therefore rusting the rear panel.
No spare time today, but I will be back on the case tomorrow for a couple of hours, and I will be checking the drainage holes in the sills.