I think i know Simon
Mine has the same issue and it comes from where the cable for the rpm meter goes into the engine. Check for a damaged cable, or the oring or the piece where the cable fits in. Some dumb ass mechanic in argentina broke mine and i had to get the piece rebuilt, a never ending nightmare, but as long as the oil leak is not too bad and you keep a good eye on the oil level, its not too bad (except fot the mess and the smoke when you stop).
Life is good!