Please don't go anywhere near an auction clutching a copy of Parkers - the traders will spot you and deliberately try and force you bidding up, traders do not like private punters.
The only people who use Parkers are private punters, it's merely a loose guide.
Get a copy of Glass's of CAP somehow, or don't take a price guide.
Chose the sort of car you want BEFORE you go, learn everything there is to know about it first, then go - give all the potential cars a good looking over before you bid.
DO NOT bid anywhere NEAR private sale price. You are buying a car, as seen, with no warranty, no test drive. You cannot even SIT in it before it goes through. You therefore need to be getting the car at a significant discount to make this worthwhile because there could well be faults.
Decide how much the car is worth to you, and do not, repeat NOT, bid above that amount. Even by £10. This is how you get carried away..