All the air and breather pipes have been inspected and found to be ok and manifold refitted.

Blanked off where the breathers enter the manifold and made no difference. I know what you mean, ive had the breather unit give grief before.

Oil filler cap is a tight fit too.

CAT has since given up and was undrivable, so opened up and removed contents and welded back together, drives fine again but still has this fault, which is clearly whats seen the CAT off. Will replace the CAT when (or if, ) this fault is fixed, no point at the moment.