If it helps, I have just sold GTI a few weeks ago using 'Wizzle' (it's free), as I did not want the hassle of private sale. Wizzle is a dealer only for sale site.
It worked well for me, car was put up for sale on the Sunday, sale agreed with a Toyota main dealer on the Tuesday, car collected on the Thursday morning. A few forms to fill in and a 5 min inspection of car upon collection, then bank transfer straight into my account. V5 done, keys handed over and drove off. Job done.
According to CAP I got pretty much middle ground between trade-in vale and private sale value, certainly I got more than I was offered by two garages as a trade-in. I was going to use Tootle but went bust a few month back.