So..... after redoing everything a 4th time I took the head off. Testing the valve tension (by blowing in the exhaust port, and then the carb inlet port) it was apparent that air could flow freely with the valves 'shut'.
In my over zealous and excited state when i sat on the finished thing, i gave her some wellie, when i should have run her in for a few hours first
It appears the valves have bent from impact with the piston, and logically would make sense seeing as they have bent upwards (towards the head).
Quick final question....
whats more likely....bent valves OR poor quality/milling of the head?
JB