Gary.
Not to gainsay any of Gareth's sage advice, but something to bear in mind with a private purchase is that you have little recourse if some major component proves to be defective; whereas a reputable dealer will either stand by any guarantee or be subject to legal redress. Only you can decide if you are willing to pay higher dealer prices for the sake of that extra confidence in the car. If you are not familiar with assessing second hand cars yourself you would find a competent assessor like the RAC would be very worthwhile.