I have had the same issues with the diff although its been covered quiet a bit in the forum couldnt get to the bottom of it, as a last resort before I bought a different car eventually sent it back to TD who said they couldn't find anything a miss with it in either the way it was built or the parts in it. Adam rebuilt it and fitted a new circlip they had been testing but I haven't bothered trying it since it was sent back as I had bought the gear diff and am happy with it.
Although it could be the diff that's slipping you may want to check the spacer that goes in the gearbox (crunch spacer) as I also had an issue with the slipper slipping to a point where I had to run it tight and put 2 nuts on the slipper shaft as the slipper would undo itself in the mifdle of a run. I put a new spacer in and still have 2 nuts of the shaft
and it seems to have solved the problem