Too late, you've already agreed to buy at X price, unless you're prepared to lose your deposit what incentive would there be for the seller to accept less.

To haggle effectively you need a back up plan and be ready to walk, with a car as common as Focus you could probably have 500 alternatives, you make a fair offer, if they don't accept it you walk away and buy one of those other 500.

It might still be worth losing your deposit if you could get the same car for £1000 less elsewhere, but you must negotiate to a satisfactory final price before signing or putting a deposit down.

Check the terms of the sale you've agreed to, pulling out might leave you subject to further charges for any losses the dealer can think up, course its entirely possible the dealer might let you cancel the deal and return your deposit for some reason, you can but ask.

As others have said, a line has been drawn in the sand, and you agreed to it. Their timing was a little unfortunate for you, but there it is. They MIGHT have waited a week and told you about a £1000 drop .... But remember that almost any car is a wasting asset once you own it. Presumably at the time you agreed the 8500 you thought it was a good deal? All you can ask is for a few freebies like carpet mats, touch-up etc.