A lot of schools give people data on first SERIES passes when asked for first time passes. At my IR school, they published a list of all the IR results for that year and while I never added them up, it looked like around 30% of candidates were getting a first time pass. I believe that that is more representative and I have heard the same number from another large UK FTO.
Anyway, getting a partial is a huge achievement in itself - it is a nightmare test and everybody makes mistakes on it - it is just luck if the mistakes you make don't make you fail a section IMHO.