Hi, Nige i am sorry if this is nonsense post put you say you have checked the turbo boost actuator operation does this mean you have check boost control valve just a thought because i have done 2 of these with very poor performance the way i checked this was by using a scope & vacuum gauge whitch was T into pipe between valve & turbo. Watch time base change on the scope & vacuum changes with it.if no vacuum change then siezed valve, we also noticed that that this was found that testing on road test was better.I hope this not granny sucking eggs Darren.