Also changed the rotor but still no spark.

Umm, so I'm crap with electronics.

I have a pertronix type installation and have wired it in accordingly and installed a GT40 Bosch coil (yes just the GT40 type - no R or T after the 40) minus the resistor (the white ceramic block about the size of a AA battery)- I am getting no spark on the plugs but when I remove the main cable from coil to dizzie, it produces a spark in expected rythm with the hall sensor picking up the 4 90 degree triggers.

So, I've changed dizzie cap, cables and spark plugs. No dice.

Anyone got any pointers? Even if I put the spark plug on the end of the main coil lead there is no spark?!?

I think I may have an intermittent ignition switch problem, do you agree? The ignition barrel isn't particularly tight and the car is rough as buggary so I wouldn't be surprised if this is the case.