I've always found www.autotrader.co.uk to be best for selling cars. I managed to get a lot of interest and a good price for my F reg VW Polo when I sold it four months ago.
It is a bit expensive (around £7.50 for 2 weeks) but well worth it in my opinion. I sold the car within 24 hours (with 2 other people expressing interest). Since it's online and has a good rep people from a fair way away will see it, and nowadays many people aren't afraid to go a little further afield for a car than just their local area. The guy who bought it from me lived 40 miles away.
I would say though if it hasn't sold within the first fortnight that you should either try lowering the price, or trying a different approach. One of my mates put a note on the local Post Office's notice board. It took a little longer to sell, but it was cheap to post the note.
If you're looking for a quick sell then definately try autotrader.co.uk
Make sure that car's immaculate when you take the photos for the ad and when people come round to view it. You wouldn't believe how many people don't bother giving their cars a wash and vac when selling them. If the car's service history is abit patchy, consider servicing it ready for sale and put a fresh MOT on it if it's got any less than 8 months left. Don't worry too much about the tax as that's always a 'perk'.
Finally, avoid Ebay unless you can't shift it as they don't go for much on there and you won't get anywhere near your £1200 asking price. As mentioned above, Autotrader's a pretty safe bet but also consider classified section of your local newspaper (weekends are best).