Hi Sam,
Not sure of the engine code with this vehicle but I had crankcase vacuum problems last year caused by a faulty vacuum valve in the inlet pipework. It was located in the small(6-10mm) pipework under the inlet manifold, I think it was green in colour. The important bit was it had a circular blanking disc inside returned by a small spring, looking at it, it appeared that due to vibration the disc wears down on its outer diameter until finally it is too small to consistently block the orifice. Hence sometimes it's ok and sometimes not.
Shaun