Ben, I'd think that they're going to look at marks across the board (pilapt, maths, physics, verbal and numerical reasoning, group exercises and interview). There's probably no need for a quantifiable pass mark as they'll likely have a fixed number of places for final assessments (based on available slots over the allocated number of final assessment days). Realistically, everyone who gets to the final assessment will have performed very strongly in every individual area. It's then down to who can make themselves stand out from a very strong crowd.
This could vary depending on the chosen FTO/ATO; I think I'm right in saying that the CTC and OAA assessments were split over two days/sessions, whereas FTE was all done at the same sitting. Please correct me if I'm wrong, though!
The tests are tough, but actually enjoyable! You can prepare for almost all of it in advance. Doesn't make it less nerve wracking, but you're more likely to do yourself justice. It struck me that there were a high percentage of seemingly excellent candidates last year. So much of it must have come down to very marginal differences, certainly in the latter stages.
Many of those who didn't get to the final stage were offered places at the flight schools (self-sponsored) and a number who fell at either the final or penultimate hurdle then went on to land cadetships through other airlines (personally, I know of four). Possibly a good indication of the quality of many of those who were in the mix. Not that everyone would necessarily be able to finance the other options, but still.