Still ongoing
Customer supplied a used set of injectors c/w rail and that has made no difference

Exhaust pressure test proved ok

drilled into the front pipe and took pre cat emissions tests well clear of the O2 sensor

at idle
CO 0.11 HC 560 C02 7.31 O2 11.6 Lambda 1.986

Fast idle
CO 0.09 HC 296 C02 4.03 O2 15.83 Lambda 3.488

I have confirmed timing with KTS and compression is good
Lambda fault keeps returning when the voltage gets to 0.2v Again, if I squeeze the return pipe and raise fuel pressure to 6 bar the O2 switches ok