Well, I finally fitted my refurbed fuel tank back in to my 2/2. Evaporust did the trick over 4 days inside, turning occasionally, looks brand new inside. I then flushed it with water, dried it fully using a hair dryer and long flexi pipe to get it right inside, then a Diesel flush to prevent flash rusting.
I did a little underseal touch ups to the boot floor whilst it was out, nothing bad under there but I guess when the restoration was completed, they had not bothered with the fuel tank.
As my fuel feed and return were the same size 8mm, I have put a smaller pipe (4 to 5mm) at the end of the return which should increase the rail pressure slightly and help refill carb bowls faster.
All back in now with new pipes clips and repainted. It's only got 20 litres in so far without leaks, but will slowly increase the volume until sender and filler are covered and get confidence that I have no leaks!