Thank you for the opinions, helpful as always.
The survey was just a hull survey, so the surveyor didn't look inside the boat.
To me it looks like either Denso tape, or fibre glass on the tube, have not crawled that far in between the fuel tanks.
I suspect the Tony is correct, the whole thing just moves around in the deck, so might have worn the deck plate. The boat is needs to come out of the water for blacking soon, so I will have to investigate further.