Thanks for the reply, I haven't run it in the dark so will try that tomorrow. I can't see it being stale fuel as I emptied the tank before filling it with fresh and have put about 40 litres through it so far.
We have tried all sorts today though so maybe some of this may help.
We've tested and correctly gapped the crank position sonsor. Tested the throttle position sensor, tested the water temp sensor. All seem ok. Cleaned up all earth connections and removed/cleaned coil packs/connections.
We've removed and cleaned the exhaust back pressure sensor as it didn't seem to be reading correctly and it turned out to be full of carbon build up and didn't look to ever have been removed! Unfortunately this didn't fix the problem.
With huge thanks to a fellow ribnetter (they know who they are) I've plugged in the diagnostic software today and found/done the following....
There are no logged fault codes except for three we intentionally triggered when testing things.
We checked the timing then performed the timing calibration in the diag software.
All plugs seem to fire when tested from the diag software.
All injectors click when tested from the diag software.
When the engine runs it missed for a while then seems to drop ok for a while then start missing again. If when the engine is missing you either disable the injector or pull the plug lead from cylinder 5 everything sounds much better. I'm not sure if this indicates a problem with cyl 5 though or whether this just drops the engine from 5 to 4 cylinders and balances everything out.
I say the above because if you pull the plug lead from cyl 5 then disable cyl 6 injector on the opposite side it makes NO difference to how the engine runs, almost as if no 6 cyl isn't firing at all! If you disable no 2 or 4 injector the engine stalls immediately! Ran out of light before I could test further disabling 1 and 3 to see what effect that had.
So to conclude it's almost as if no 6 isn't firing at all and the even things up you have to stop no 5 but what I don't understand is surely disabling 1 or 3 would have the same effect?
Either way all of this just helps along the way and we haven't actually found the problem yet......sigh
And if anyone has actually read all that is still with me, well done.....any ideas hugely appreciated at this stage.