Stripped out the PCV / vac system from under the inlet manifold and crankcase breather etc. It was full of gunge and taking pipes out I managed to get it all over me and the engine. Crankcase breather valve and suction jet pump were rammed full of it!
Stripped it all down and cleaned out all the crap. A couple of the pipes had seen better days and gone all soft inside, so these will be replaced next week.
After all the effort, there's been f#$% all improvement to anything I was trying to sort out.