is a good place to look to get an idea of the value.
There are some 50k mile '02 cars on there for £6-6500, so I guess what Ian said "Whatcars" value was seems about right.
Have a bit of a hunt round yourself for prices - it will also depend what you are willing to accept and how fast you want to sell. Something that low milage may be worth £7000, but you may have to wait a few months for the right buyer, where as £6000 might get you a buyer next week.
...I guess another consideration is if you still owe anything on a loan for the car which must be covered by the Sale
Have a look at the prices on Autotrader, ebay, Pistonheads etc, but I recon £6200-£6800 sounds pretty accurate