Hi Nigel

I have only just seen your post , these engines do suffer with the crank case breather valves failing causing the symtoms you describe. They are situated on both rocker covers ( 1 each bank ) and are a cliped on plastic disc clipped onto plastic moulded rocker cover under the main coil covers.
These are not expensive and really easy to replace.
Hope you get it sorted , if you need any more info let me know.
Kind regards
Steve