Driving home happily from a trip to the coast, luckily on a B road, engine started running on 3,pulled in a side road, took the plugs out and found no.2 oily. Cleaned it and put it back in, spun it over, and a big spurt of water came out of no. 2 plug hole. Game over.
The builder said it had a cracked head, which was strange as it was tested before the rebuild as i bought it off a racer so was never fitted to an engine in my ownership prior to the build of the 1500.
Steve