Thanks Chris exactly what I was thinking, where is it situated on this engine as it dosent have the regular unit which bolts to the rocker cover?

I suspect it is actually part of the cover itself as there is what looks like a moulded valve within it which has a rubber diaphragm inside.

Am I right in believing this diaphram should only allow flow in one direction like the other types?? This one allows flow both ways so I am thinking this could be our problem but at £120 +vat I'd like to be sure I'm unsure of its purpose what with this being non turbo and if failure of this valve could even cause this issue on this engine?