Problem is fairly big, but hopefully confirmed

A few more tests to verify what was going on showed for 100% certain it was Pot 4 at fault. Haven't compression tested properly, but with plugs out and thumb over the holes it showed that there was little or NO compression whatsoever in that cylinder.

So that is the one for sure. Of the the many different reasons you can read on the internet, and after advise received, the most likely I feel is a burnt out / broken valve. Checked the valve clearances and that hadn't been knocked out somehow, just in case it was simpler after all, but no luck. It still points towards needing to have the head off, even if the reason is different.

(Spoke with the chap I bought the car from and went through the symptoms, the lack of constant steam, the lack of emulsion in the rocker cover, the fact that the car still runs after a fashion all points away from the cylinder head gasket being the root problem. Plus the gaskets apparently tend to go BETWEEN two cylinders, so you'd have two with low compression, not ONE with NO compression)

I have a full spare set of valves from the old engine and a new Head Gasket and full Gasket set on order.

So I am hoping that a couple of weekends, firstly dismantling, diagnosing for 100% final confirmation and then grinding and re-assembly will get me back to square one. Maybe even better, because that 4th pot was always blacker, which with hindsight probably indicates lower compression in the first place.

We'll see!!