Bongo Fury

Update.
I drove to the garage and after the drive and a bit of idling, it sarted to splutter at 2700rpm. They looked over the engine, but with there being nothing obviously amiss, they have suggested a diesel specialist, but suspecting air being drawn into the system somewhere. We left it alone for 10 mins and when it was restarted we could not get the syptoms again . Feeling like maybe the hand of god had fixed it for me I left and went on another 10 mile drive. After which, the syptoms presented themselves again - this time not missing / hesitating until about 3400rpm, still with blueish grey smoke ( definitely not black or white). At tick over and even when being revved (up to the hesitation) it is as sweet as a nut, with no smoke at all.
I havent put the wrong fuel in, but could have had a bit of muck in the system, and I am confident its not the manifold studs - no squealing and the noise was only at idle where the belts were slipping.

I have cleaned out the banjo filter (was a bit mucky, but not too bad), and running some injector cleaner through it (well there has to be some magic fix that you can pour in ).

This is about all I can do as the specialist cant look at it for a few days and me and my son are supposed to be camping / kiting on the weekend and I dont fancy sleeping in a bare van or an awning.

the fact that it sometimes returns to normal means the engine is ok. if it was a problem such a holed piston broken valve or anything like that it would not ever be normal .
i would suggest that it is fuel related and if it only occurs after a while possibly a blocked fuel breather starving it of fuel, that is easy to test just open the fuel cap and listen for a large gush of air, if it then runs again ok for bit then check fuel breather.
Or as suggested before check banjo filter and main fuel filter.

Ed... You are describing EXACTLY identical symptoms as myself and daveblueozzie experienced when we picked up some diesel with traces of water in it... There were 3 of us in convoy, Dave and i fuelled up at the same place, then maybe a 100 or so miles further on we both lost power at exactly the same moment, then the thick blue smoke, then the engines wouldn't rev past 2000RPM with barely enough power to pull away from standstill.

We messed around cleaning banjos,fuel filters etc etc maybe 30 or 40 times on each of our bongos. Each time the symptoms seemed to have decreased, or disappeared, till we realized that it was simply allowing the engine to cool(whilst faffing with the filters) that seemed to help with the smooth running.

We both ultimately needed recovery home. The fuel tanks were emptied, removed,chemically cleaned, cleaner was ran through all the fuel lines, Brand new fuel filter fitted and filled with neat injector cleaner upon start up, and of course a fresh new tank of diesel...Mike at Wheelquick said we were lucky not to damage the rather expensive fuel pump! I'm not saying that's definitely what's wrong in your case, but the similarities are uncanny!

EDIT: We even disconnected the fuel intake pipe and connected it to a fresh source of clean diesel with no improvement, upon strip-down the injectors were covered in the sticky white mayonnaise that's evident when the cylinder head gasket has blown. The ultimate problem lay with the injectors being i'd say 80% blocked.

The very same thing happened to me recently (although in a car, not a Bongo). Problem was gearbox oil, or rather the lack of it. Drained old oil and refilled = everything okay again.