Hi Loz,
NATS generally keep quite quiet about how the whole thing is marked so it's hard to know for sure. What is known for definite is that you are not competing - at every stage there is a set standard and every applicant who reaches that standard will be accepted on to the next stage.
If I were to speculate (and it is largely speculation more than anything else) then I would say that I think that at stage 1 there is a pass mark for each individual test and you have to pass all four tests to get through. I think the pass mark varies significantly for each test, for instance I've heard of people who claim to have got as little as 50% in the cubes still getting through, where as I believe the pass mark for the other papers is a fair bit higher than this. What the exact pass mark is for each paper I couldn't say.
The waiting is the worst bit, I know I spent ages after each stage analysing how well I did and worrying myself silly over every potential mistake, but there's nothing you can do. Just don't worry about it and try not to get too caught up in checking your emails several hundred times a day. 15 days seems like a long time, but it'll pass. If you think its bad enough after stage 1 wait until after stage 3....