I used to buy cars from auction for a living in my previous job. I mainly used to buy nearly new/low mileage vehicles, sometimes almost 50 cars a week!
There can be some real bargains out there (I bought my ST200 from a BCA auction nearly 2 years ago) but at the same time there are some dogs.
Best bet is to check out the local auction houses a few weeks before you buy, and only bid on cars that are entered in by lease/rental companies rather private individuals.
Auctions houses have to HPi every vehicle now so at least you get that piece of mind, it's just the mechanical risk. Try to stick to BCA or Manheim, their the biggest, therefore the most reputable.
May2002, if your in Leicester check out Manheim Auctions @ Shepshed on a Thursday, they have a Vauxhall Master Hire section there most weeks.