So, to be fair to the local Nissan dealer, they waived the cost of recovery from St Alban's and their diagnostic fee.
Had a quick look at the car, and think their diagnosis might be a bit excessive... Although I understand the mentality of throwing a new engine in, there's the possibility that a new head gasket might cure it without much else.
Basically their diagnosis demonstrated the following:
1. The cooling system is both contaminated with exhaust gasses and losing coolant via the exhaust.
2. It depressurises into the cylinders when cold causing the partial hydrolock effect.
3. The car has recorded 136° max coolant temp.
4. The oil cap is covered in caramel mayo.
Item 4 is a red herring, basically the car does alot of shorter journeys with the occasional longer one so never really warms up properly. The oil on the dipstick looks like it has been in since November, but absolutely zero evidence of water mixing and the oil level is spot on.