If the tests are done correctly then it is a good chance that U2 or U3 is bad.  Also try it with the game prom removed. and test the IRQ then to ensure this is not dragging it down.

I will do this now and report back.  I think between me learning how the logic probe works and my nice new shiny DMM I believe I've done everything right as per Clay's guide. 

I've emailed Ed again - asking very very nicely if he has a QuickScan tucked away somewhere (shop listed sold out) of his QuickScan's....Just in case I get a HH or another Sys80 type game...Plus probably handy for someone locally to have one in case others get into trouble!