I have persisted with the 4mm sheet , and after annealing as mentioned above , achieved a reasonable result which I am going with. This involved a few annealing sessions , using a lump hammer and my blacksmith vice to get the correct shape , but got there in the end. I have new respect for how much punishment aluminium will take. A trial go on an offcut helped get the right shape/angle which is surprisingly shallow - I didnt realize how much the tanks follow the angle of the bulkhead.
Did you chaps allow for a gap between the new bracket and the tank for the glue fillet , or just go on the basis it will flex as required ?
