Okay so here's the thing.
Just a bit of general advice :-)

If you are trying to get a filament feed tube out of one of those weird tube holder things.
The things at the bottom of the pic:

DO NOT GRIP THE TUBE WITH PLIERS !
Particularly if you're using 1.75mm filament and the tube has internal diameter of 2mm.
Finally tracked the issue with the delta down to this. Basically I've buggered the internal diameter of the tube and the extruder can no longer push filament past where I used the pliers.
The hotend and nozzle side of things is absolutely fine.
New tube arriving today. And some spare connector doodads as I can't get the old tube out of them any more. Not even with pliers :-)

So tuesday's lesson is:
Never assume, you know FOR CERTAIN, what the problem is :-)