Try contacting them here:
Phillip Dutton Eventing
Explaining your situation politely, that you are looking for your horse, etc. They may have already sold or retired him, and may be able to forward you more information about his current owners. Be POLITE! Say hello, explain with good grammar, and thank them. I would also leave contact information.
This thread by Orange mentions him at the 2001 three-day
Simply Red - Eventing horse
2001 three-day info:
Carimati Cup - Phillip Dutton/Simply Red (AUS)
Best Conditioned Horse - Simply Red/Phillip Dutton (AUS)