Part Exchange Value - Dogbreath
Whats the best guide to what the dealers will offer me? "What Car" "Parkers" etc seem well out. Should I take the price in Auto Trader and knock off £1000/£500?
Part Exchange Value - Twin Turbo
The BEST guide is what they will offer you. For an indication Glass's Guide are used by the Trade and can be found online, The Daily Telegraph's motoring web site has one.

With the Autotrader route be careful private cars could be low and trade high, good luck.
Part Exchange Value - DavidHM
What's the car, the condition and the mileage? Colour, spec and history matter too, espeically on newer and more prestigious cars.

As ever, you're better off selling privately if you can, although not everyone wants to do this. In the end though, what you are offered is a bit of a lottery; if there is lots of profit in a car or the dealer is desperate to shift the one you want, he'll give you more than if he only has £200 of margin in it.
Part Exchange Value - SteveH42
A lot depends on the car you are buying. I was offered anything from £200 to £1700 for my Tipo when trading it in - most dealers build in an amount in the screen price to allow for a trade-in, but if the car you are looking at has been sitting on the forecourt for a few weeks then they are more likely to give you a high trade-in than if it has just come along and they know they are likely to get the full screen price for it easily. The Parkers etc prices are a good starting point, but until you actually talk to a few dealers you can't really tell I'm afraid.