Oil leak - roadster V8

[IanVFR750]

I have had a bad oil leak from the rear sump / gearbox area of my car but it is impossible to see where its actually coming from.
I recently got round to fitting a new sump gasket.
This did not solve the problem so I tweaked the sump bolts a bit but no improvement
Can anyone tell me if there are any known suspect areas for oil leaks between the gearbox and engine block.
A mechanic friend mentioned it might be coming from the crank seal.

Any details of similar problems / resolutions would be much appreciated.