FIXED at last.It was the fuel. Sucked it all out by removing pump/sender unit. Blew out fuel line & filter, transferred 20 litres of known good fuel from another vehicle. This is returnless so difficult to clear every last drop. Dried plugs & blew out chambers. Just as before it was reluctant to start and smoked heavily but eventually it ran, gradually it improved as new fuel flowed. Smoke was still heavy for a long time but lost smell. Water vapour/steam was then the problem, just like a bad head gasket but no other symptoms. Can only assume silencer was full of water, maybe from contaminated fuel. Yet to drive it but after an hour running vapour has almost stopped.
What was in the fuel? Don't know.
Twice prior to this we have drawn off at fuel filter good sized samples.
It smells of petrol.
It dosn't smell of diesel.
The smoke dosn't smell like diesel smoke.
It looks like petrol.
It dosn't have visible water seperating.
It burns vigorously.
If spilt it leaves a wet residue for several hours(didn't notice this until today)The residue has no discernable smell and looks like water.This must be what was fouling the plugs and creating the smoke.
Customer previously said he filled tank, was down to 20 litres by the time all cats had failed & blocked exhaust.Despite clearing cats & diconnecting silencer we never got it to run well enough to drive after that.
The huge injection pulse? I can only assume the ECU must have a strategy that injects ever increasing quantitys of fuel if the engine fails to respond to throttle when cold starting.
Thanks to all for ideas.
Phil