Took a look at the inner arch today with the usb camera.
The reason for the rust became apparent. When the conversion was carried out the scrott that cut the opening for the water filler didnt clean up the swarf afterwards. The filings and cuttings are what started the corrosion process.
Annoying as I will need to take the side kitchen,wardrobe and bed out to get in there. I have flooded the area with Fertan for now.
How simple would it have been to do that at the time
https://goo.gl/photos/j3dTmdKfq2CFRMhM8