UPDATE !!
Vehicle has been fixed yehaaa.....
Please bare in mind at no stage during the the fault finding process did this fault register at all with the (EML) Engine Management diagnostic fault code readings etc In fact the EML was never illuminated apart from when ignition is switched on!.....Defo one to look out for.
Yesterday the fault was traced to the crank shaft sensor which was smashed to pieces ! With the help of another mechanic we was messing about with the loom from ECU to engine to which when moved and jiggled around made the fault appear and disappear. We stripped the loom to thoroughly check all the wires but found no breaks or chaffing etc.... And then there it was, branching off the loom, beside gearbox housing is the wiring that reaches up over the gearbox to the crankshaft sensor, as we pulled it we actually felt the Crank sensor move! Grrrrrrrr....... The sensor just pulled out freely from its location, the plastic moulded bracket attached to the sensor to which the bolt goes through to retain it in the bell housing was broken and not present, neither was the bolt. The guide/location bracket inside the bell housing was also broken! The end of the crank sensor was chewed/smashed back to the magnet sensor. I'm amazed it ran at all to be perfectly honest.
The Fix..... (thinking outside the box)
New crank sensor from euro car parts £26
So the fix would have meant removing the gearbox to fit a new location bracket which the bolt goes into from the new sensor.
I really didn't have time for this now as this fault has been meant my truck has been off the road for a few weeks.
Using two brass washers we trimmed them down with an angle grinder to mirror image the base shape of the sensor, this allowed the sensor to seat correctly and space it accordingly, Now to secure it without it popping out or turning around!
For this we used a soldering iron to indent/mark approximately 1/4" all the way round of the hard plastic shaft of the crank sensor of where it passed through the bell housing thick metal, thus changing it from of smooth slippy surface to a knobbly golf ball rough like surface.
Pushing this home it felt very tight and secure. We then set about tapping up the loom and fitting a new bracket to the gearbox to simply secure/cable tie the loom thus minimising movement to the sensor loom etc.
Boxing the vehicle up we then started it. On the button, no flashing (----) miles to empty display, No fuelling issues on live data, revs perfectly with no faults stored at all on the ECU....
Took it for a road test which involved me going for bumps, pot holes, drains etc in the road and it drove perfectly for the duration of the 15 min test. However the big test is today as I will be travelling 200 miles but tbh I know the problem is fixed due to the severity of the sensor. I will report on my road trip later.
Really hope this helps others in the future.... Goodluck fellow transit owners