Update:

After a full top end strip down, the cylinder head gasket appeared to be severely split between cylinders 1 & 2 (area of about 7mm x 15mm), but there were no signs of the missing section anywhere in the cylinders. Excessive amounts of liquid gasket/mastic on the mating faces of the head & block, along with missing gaskets replaced with said sealant goop and several other things including missing bolts, tippex timing marks on the cam sprockets (even though they are adjusted when the cams are locked? Pointless?) and copper K seal type crap in the coolant shows signs that the head has been bodged previously.

Replaced all gaskets and head bolts (althought they don't neccesarily need to be changed - didnt trust the previous by the state of the rest of the stuff) skimmed the head just to be sure and timed the engine up to what seem to be correct and the coolant is no longer pressurising and the pinking seems to have stopped. drove steady and no more than 2.5k revs on the way back but up the hills doing low revs in high gears it pulls well and is pretty quiet.

So far so good, will add some more information later after I've taken it for a good run to make sure the ECU has recalibrated correctly.