It's tempting to say 'it's worth whatever someone will pay for it' - but while true that's no help to you!

You haven't said how old it is - but I'd suggest looking on Autotrader and seeing what Focus estates of similar age, mileage, condition and equipment are going for.

You'll get more if you can be bothered to sell privately than if you go to a dealer. Sure-sell (sponsored by British Car Auctions) have been highly spoken of on here: I tried them a few weeks ago to see what my Y-registered Z3 would be likely to fetch, and they wouldn't put more than £3,500 on it as a reserve although privately it would go for at least £5,500. They potentially take the hassle out of selling your car but you won't get much for it.