Tactics

Hi
I was hoping for some expert advice on purchasing a used car from a dealership.

So, I walk in, see a car I'm interested in and the first thing I'm asked is have you anything left to pay on your current car.
Say there is currently around 5k left to pay. What do I tell the sales person? Yes there is still a balance to pay but don't let them know how much?

The second question has probably be asked a million times but how much can you expect to get off the advertised price say on a £12k car? 500, 700, 1k?
I guess this also depends on the result of part one of how much I get for current vehicle.

Any tactics and advice would be welcome as I always feel like in getting ripped off and would like an idea of boundaries .

Cheers