Thanks for your replies Lex, bwa.
The problem with a T thread adapter is you don't get the benefit of using 2" filters
Had a look at TS, they sell an M54 to M48 adapter and M48 threaded 2" extension tubes, which, I think, is the way to go.
I'm thinking of buying a Tak FSQ 85ED which has a built-in flattener, so I could plug the extension tube straight into the scope.