Thanks guys. Engine has just over 2 gallons through it now. Was broken in with the run in shim fitted, which I removed after 1 and a bit gallons. The plugs that come out look good. No distortion in the coils that I can see. Coils are nice and silvery, and plugs are slightly damp on the bottom. Stopping engine by flywheel method and occasionly by pinching fuel line to check LSN settings. I have changed back to the standard head for the time being, as the turbo plug count was gettiing a little expensive.
My other 5 Port which is newer (about 1 gallon) and running standard head at this stage rarely blows a plug. Broken in same way, same fuel etc. I might put the turbo head on this engine and see what happens. Cheers