You need to understand 3 figures in advance. Have a good think about them in advance because indecision during the negotiation is a salesman's dream
(1) Your target price. This is your 'best outcome' price, and is typically on the optimistic side. In this case it could be £11k or even lower. Know your market and know the real value of the car.
(2) Your acceptable price. This is typically a range rather than a single value, so it might be 'between £11k and £12k'. This is likely the range where you will agree a price with the negotiator. Try and understand his target price and see where he's edging you toward.
(3) Your 'walk away' price. This is the hardest one to decide on as it is, as the name suggests, the price point you will actually leave the garage empty handed. It's really important to decide this before you begin; never change this during your discussion. If the salesman simply will not agree to lower than whatever price you've decided is your walk away then politely explain that you'll have to take your money to a competitor but thank him for his time. This can be used as a gambit to force his hand but is very risky. He has to believe that you're walking away and taking your dosh with you.
There's loads more I could write but this is the gist of a negotiation. Ask some probing questions to see what buttons get him going (finance? cash? deposit? payment in full?) and focus on those. The idea is to walk away with as much as you possibly can having paid the absolute minimum you can.
Be polite and interested in the car but not so much as to suggest you simply have to buy it. Being keen is fine but being desperate is not. If you can throw in something along the lines of "I really do like the car, great condition and I'm pretty keen to sort something out this weekend but I can only afford to go to <insert target price>", this will start the negotiation. You're going in low, probably unrealistically low, and his job is to get you up quickly and yours is to inch up or stall.
Finally, believe and understand your three numbers mentioned above and don't be afraid to walk away if you're not feeling good about how the negotiation has gone. Good luck.