Cheers for this Paul, I did watch and read a few articles about back pressure and scavenging and as you say it's quite complex and ideally you have a exhaust built and tuned to your engine.
In the end I went for a 2 1/4 inch pipe for my 2L Rover T Series engine, it seems ok but it's blowing slightly as only clamped atm with no exhaust paste, I will eventually get bits welded but want to make cat removable.