I think you have solved the mystery Jim.
A little voice at the back of my head was asking 'why does it get harder to fit each time
Yes, I have had the tank off for prolonged periods of time.
But how to get back to original shape?
I cannot get the nut off the stud to fit a smaller washer, hence the grinder as the only option.
This is a concern for future ventures also. Just how much will this tank stretch? I shall have to brace it somehow while it is off the bike. Maybe wrap the tank lengthways with a ratchet tie down strap while it is off the bike.