But that's where i get confused (easily done Grin) ... there is not a single trace of Mayo under the oil filler cap! And no trace of exhaust gasses in the coolant header tank undecided
The K is a wet liner engine, so the only failure mode for the head gasket if leaking from around the bores (where they all fail due to liner movement) is into the coolant jacket. There is no deck on the top of the block to allow the bores to blow into the oil return holes and it's very unlikely that the oil gallery feed into the head, or the oil return holes into the sump, would fail on their own.
I'd get a garage to look for evidence of exhaust fumes in the expansion tank if you're still sceptical.
These are exactly the same symptoms I had on my Rover 214 years ago. Didn't have a clue about cars then and, I'm ashamed to say, I assumed it just needed a flush, did so and then sold it to a mate of mine who had HGF a couple of weeks later! Luckily he wrote it off soon after that so he couldn't blame me for anything else.
Kevin