When I was standing around scratching my head wondering what to do next, I glanced at the banjaxed engine and thought about swapping the sensors off it. I looked at them all and noticed that there was an extra thing on it that the replacement engine didn't have, namely a Manifold Air Pressure(MAP) sensor on the actual manifold moulding just behind the throttle body.The donor car had a Mass Air Flow(MAF) sensor on the air intake and the recipient car didn't.When I looked at the replacement engine there was only a blank plate on the manifold where the MAP sensor should be.It didn't want to come off so I just swapped the whole manifolds complete with throttle bodies and used the original bit of harness with the extra plug on it.I'd been genning up on sensors and I read that the Crankshaft Position sensor was the one that triggered the spark so I pulled them both out to test them.The one in the replacement engine had like swarf or iron filings stuck to it because of the magnet.I checked them both with the multimeter and got identical readings.Waving a spanner in front of them got similar deflections on the meter.I swapped them round anyway thinking it won't do any harm and lo and behold it fired up immediately. So I don't know what did the trick - the MAP or the CP but I'm delighted it's going now!
Thanks again guys for taking the time to reply