So I've been and had a play
1.2bar is fine, 1.5 bar is also fine
1.8bar high boost I managed to get it to do it, if I'm cruising in 4th and plant it obviously it's a bit slower picking up and seems to be absolutely fine
If I hard shift from 3rd to 4th and get back on that seems to be what it's not liking
But out of the 3 or 4 times I done it, it only seems to have slipped once this time
Have checked the box again, tiny amount of fresh oil around the bottom again but inside the inspection still bone dry so I don't think it's oil contamination
I can see a bit of the clutch plate inside and to be fair it still looks like it has loads of meat on it so I'm confident it isn't worn down which I wouldn't expect anyway for 2k miles