I'm beginning to sound like a stuck record, and I promise I don't work for them (!) but I have had great results from Suppleaze Gold, and it's also one of the more reasonably priced supplements on the market that still has all the magic ingredients.
Equine America's Glucosamine 10,000 mg glucosamine and MSM added. £14.99 last for about 2 months at one scoop per day. Excellent stuff and the MSM is very good for mild bone spavin. 10,000mg of glucosamine is one of the highest (if not the highest) dosage on the market!!! Wish they did it for humans - I can only get 1000mg myself.
I have used a variety inc Naf Superflex and like to try different things but I did actually go back to Equine answers Premier Flex plus - it seemed to make the most difference with my boy but its difficult to tell........ if it was that, the weather, physio, etc etc etc http://www.equineanswers.co.uk/
Cider Vinegar is also a good natural remedy to help with stiff joints, of course I think the more up to date supplements are proven to help but I must say I have used cider vinegar on horse which suffered from arthritis along with extra strength cod liver oil, I didn't believe it would make any difference but I did, a very noticeable difference in fact.
Thanks for all the suggestions, I'll print it off and try them out. He's never been lame or stiff in the 7 years I've had him, but it's been so cold and he's getting older I think he needs a bit of tlc. Thanks for your help.