This does sound like Head gasket failure, I'm afraid. Is it using coolant? Any oily sheen in the coolant? Any ground hugging white smoke from the exhaust?

I once had a h/g failure between cylinders 1 and 2 and, under acceleration, the engine knocked badly. I figure hot gasses from cylinder were leaking into the adjacent cylinder and causing preignition. Although this H/G gasket failure did not affect the coolant system.

I agree that sounds likely and I should have said in the first submission that the coolant looks and smells just as it should. The exhaust fumes are invisible and not smelly either.

Sounds a bit like your h/g failure, though.

It does seem that the thermostat is stuck open as it over-cools at speed and on repeated short runs leaves mayonaise, but not on long runs.

Another thing, it doesn't seem to be down on power; there's not much there at the best of times so a loss of one cylinder would be noticeable and it runs very smoothly apart from the loud ticking. (it's not quite hard enough to be described as a knocking)

I wonder if it is the gasket and it's leaking out of the engine without affecting the coolant or another cylinder??

I have the same with my 1.6 8V have you found the answer?

I also carried out a compression test but that was within spec for all 4 cylinders.