Jock and Australian Kevin McNab, whose horse also tested positive for the same substance, were cleared of any wrongdoing by an FEI tribunal last month.
Clifton Promise was ridden by Jock at the Alltech FEI World Equestrian Games last week, but Jock retired after an early stop cross-country to re-route to Burghley.
“He was good and having a horse like that you would like to think you can be right up the front of the leader board after the dressage,” Jock told H&H.
“Today he didn’t feel as good as he did last week at the World Championships, but it has been tough on him.”
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