I think Dan has mentioned previously that vehicles are generally prepared for a week or so before presented for test. I believe first time pass rates count against an Operators compliance scores? I can recall reading an interview with an engineering director (can't remember which company, but it was one of the large groups) and he stated they aimed for a 100% first time pass rate, as if a vehicle failed when presented at a booked date/time with an examiner, what would the DVSA find if they pulled a random vehicle over at the roadside.