As an update...
The car has been into the garage today , they've stripped out the engine and confirmed that CH 4-6 is fecked and has been allowing oil into coolant and most probably over pressuring the cam covers causing the AOS issues I had.
A big decision to strip out the engine, it's a very fine line between whether to scrap the car or repair it as the cost of the engine work will likely be about £2.5K using a second hand replacement head. Only the fact that I've invested so much time and money into making the rest of the car so good persuaded me to go ahead and repair.
Bloody Boxster!!! Can't wait to get it back for what's left of the summer though!
