The target price on a brand new Yaris is now £7879.00.. if you haggle properly you can get one for that price.
I think you need to go as low (maybe lower) as £6500... i know it sounds "

" but i'm afraid that when you are selling what is more or less a brand new car on the 2nd hand market, the drop in value in the 1st 2 - 3 years is quite considerable. If i tried to sell my MR2 Roadster for more than £15,000 i would struggle.. and they have a window price of £17,500 new.
If you really need it sold now.. you need to be prepared to loose a fair bit of cash on the deal

EDIT~: Someone at my wifes work got a brand new 1.3 Yaris Blue ( i think.. if not it was a colour collection) for £7,000 from SDM Falkirk.