NB.*** I have now discovered that this noise was caused by the top left hand timing chain plastic guide rail in the cylinder head being broken off at the bottom. It was very difficult to see this damage without removing the camshaft gear wheel and pulling the chain out of the way. Not that difficult to fix if you know what the problem is. Hopefully this video might stop someone scraping a car or doing an unnecessary full engine rebuild.
The problem happened when I tried to rev the engine to clear a misfire caused by damp distributor caps as the car had been standing for a while. There had also been an occasional engine screeching noise which I originally thought was just the water pump but obviously not as it’s stopped now and turned into this knocking noise. Oh dear.. I should of just cleaned those caps..
