Hi Nigel, Scotty
I do not think the photo is showing the correct way to assemble the mounting???
The rubber bush should sit inside the cup washer
There should be a steel tube that goes over the T bolt holding the washer with the cutaway up against the oiltank loops
this steel tube goes through the rubber tube and then a large OD washer pushes the rubber into the cup and the nut washer tightens against the end of the tube
This provides a rubber in compression mounting for the tank
Every BSA I get to work on has this assembly put together incorrectly
Attached is a scan from the parts book that shows the correct sequence of assembly
HTH
John O R