OMFG - ITS ALIVE!!!
Plugs look very black, but all the same.
Compression tested it and got just under 9 bar on all 4 cylinders, so definately no major compression loss anywhere like I had suspected.
Whacked a set of cheapy autofactor plugs in it today and it fired up.
So it was just the plugs had got coated up with soot as I neared the end of the bottle and the pressure dropped, thus making it go much richer.
This old cossie engine I built from scrap bits just will NOT fucking give in, LOL