Ok the filament use is a tiny fraction of the requested feed, maybe 10mm out of 100. One of the extruders drive had significantly less grip than the other, so i've stretched the spring and re-installed, but no help. I also tried increasing the hotend temp from 240 to 250 and reducing these settings;
Rep host -> config -> printer settings -> printer
changing manual extrusion speed
left box 2 -> 1, right box 20 -> 1 (units mm/s)

Rep host -> config -> printer settings -> extruder
max volume per second 12 -> 4 (mm3/s)

But none of that has helped. Next i tried removing the fans from the extruder drives and confirmed that the filament is slipping in the drive, so its not a problem with voltage to the stepper motors. With the PTFE tubes removed from the hotend the full 100mm comes out. So I either have a problem with the level of grip in the drives, or resistance to flow in the hotend, or a combination of the two. I can push the filament through manually. There's a lot of resistance but i'm guessing that's to be expected as there's a solid fat filament that needs to be melted and pushed through a small orifice.

Any suggestions or do I just keep trying to get more drive force onto the filament?

Or do i give up and get a better printer? I've spend weeks on this now.