well after much swearing - throwing of tools and generally cursing - the new tank is in and leak free!
the problem seemed to be a badly fitted sender unit - and think one of the seams was FUBAR'D
silly design on the sender unit - the inlet pipe (return) is 1/2 " long to allow the tank to be fitted with the sender attatched - so a light application of hacksaw cured that!!!!
also had to remove ARB and Tow bar retainers to allow us enough room to get the tank out.
