First stop, of course, is to report the theft to your local Police Station. They'll give you a crime number which you'll need to get closer to finding your horse.

Horsewatch do some fantastic work and locate quite a few horses. They work Nationally and each branch communicates with all the others very efficiently. Your nearest branch can be found here: http://www.ukhorsewatch.org.uk/Map.html

The National Equine Database is becoming a valuable resource for locating stolen horses and you should contact them next http://www.nedonline.co.uk/public/Home.aspx

If you give NEDs your crime number, they will feature your horses' details on their website. They hold all the information for every horse passport in England, and should an application be made at some point in the future for a new passport for a horse matching yours, they'll be the first to know.