I think you should look at Equine Americas 10,000mg glucosamine with MSM at £14.99 for 60 day supply. Very good value for money and the MSM included in it is particularly suitable for horses with mild bone spavin.
I used this for ages and it cost me a fortune, so to save costs went onto cortaflex. There is a huge difference in my horse since I went on cortaflex, he is so much better.