All 6 nylon captive fittings had pulled out of their respective holes which left the fuel tank holding on for dear life.
Only the filler neck and a bespoke exhaust hanger bracket was holding it in place.

All the nylon captive fittings / screws are brand new which begs the question, why have they failed.?
Anyway, after some head scratching I have decided to adopt the belt and braces method of holding the tank in place by drilling through the original 3 mounts for the tank on the rear subframe, rear crossmember, into the load space and using long bolts attached to a spreader plate etc etc.
Then I'll manipulate a steel bar to go underneath the tank front to rear, a "U" shaped bracket idea which again will be bolted through the shell.
Has anyone experienced this sort thing before, or am I just unlucky.?