job done
as you can see from the picture the story has a happy ending. The panel beaters put some heat into the leg, and it behaved well and went back to where it should have been. Not surprisingly the tank now fits perfectly, and there was no damage to the external finish, which is now putting the rest of the bike to shame.
I now have a white 1150GS tank for sale if anyone might be interested. I will take some pictures and advertise it on here.
oh no my bike is upside down!