I noticed last night my Sapphire has started to smoke a bit on overrun,taking it out today and it seems ok at start up but again quite a bit of smoke on overrun and giving it a rev from idle once i stopped it gave a few puffs and seemed to clear at normal driving speeds.

A bit of googling suggests valve guides maybe stem seals or the turbo could be on the way.

Being new to the Cosworth engine what's best to start with?

It's meant to be on a stand next weekend at the McRae rally challenge so I could do with it being something simple I hope.Being an old Ford though I doubt it haha