I know that your title says that Best Mate dies, but in racing terms, that can mean, was in the lead and then went to the back.
I did not get from your post that the horse had actually died.
When I saw that the horse was running in a 2 mile handicap chase, I could not believe it.
Best Mates racing distance was in excess of 3 miles, so he was having to run nearly twice as fast as he was used to just to keep up, and he was carrying more weight than the specialist 2 milers.
Such a tragic end to a special animals career.