I took the tank to be repaired yesterday and it duly was and pressure tested all as I observed the proceedings. It was the seam at the bottom the tank in the region of the petrol tap. Not too much damage was done to the paint work but it is still going to be a bit of a challenge to make it invisible - think it will be beyond my skills to make it invisible.
One thing does concern me however, and that is how to get the tank really dry after treating it prior to the liner and also how to get the liner both fully swilled around everything as well as draining out any excess. Unlike the 1951 NH tank which is straightforward shape, this panel tank has about 1cm if much between the centre ridge and the top which is quite restrictive because the filler is way off to the left, not straddling it (as for the 51 tank). Also excess liner is not going to drain out of the tank filler because this reaches below the top of the tank since it protrudes below the recess for the instruments. I'm sure someone out there has dealt with this and I'd be interested to know how!