Just done a compression check and cylinder 1,2 and 3 are at 175. Where as no.4 is down to 100 - so this must be the fault. Firstly what could be wrong? Poorly seating valve or dodgy piston ring? Causing oil to come past the piston or something? But then I would have thought it would be smoking all the time.

It has recently had new valve stem seals, still runs well and is quick, so turbo is ok in that respect.
Can i get an uprated breather system or do I need one? And if so what exactly does it do? The metering unit is getting oily - as is dumpvalve etc. Any more help from anyone out there?!?!