Jilly, try Howarths, they will give you an idea
http://www.howarth.uk.com/
If you have an active ebay account with good feedback I would put it up there. Expect anything from £1200 to £2000+ depending on condition, it looks good to me, but get a report befoer selling it. One sold about 6 months ago on ebay for about £1200 but it had no indication of playability or very good photos.
If Howarths say that it needs work, then take their advice. It might be worth spending a few hundred £ to achieve an ever better price. A good repairer for theses (if Howarths don't want to) is SH Woodwind
http://www.shwoodwind.co.uk/Reviews/Saxes/Alto/Grafton_alto.htm
Good luck with it, you will have NO problem selling it, but to achieve the best price I would ebay rather than go through a dealer, but it is up to you. Or, you could post it here on the marketplace once you have had it checked over, but it is not an auction,so you will have to state your price up front. But it is free! (although it would be good to pay a donation to the site)