Well what do you know? I got the same chance today too!!
I got the chance to ride a Prix St Georges dressage horse that my instructor owns. I got on so well with him (and I'm told nobody else except his owner has gotten him to go as well as I did!!), that I am to compete him at Advanced Medium, ride him twice a week, and get dressage lessons on him with a paralympian! It was amazing, and I did travers, half pass, simple changes, flying changes, shoulder in, and medium tort (his mediums are MASSIVE!!!).
Its amazing isn't it?! Schoolmasters are by far the best teachers...
After riding this horse, just once, I had major breakthroughs and a beautiful ride on Diddly, who is unbalanced and green. Diddly did some lovely leg yields and shoulder fores, and felt much more balanced, and I knew what to look for as I was using my core... I had no choice but to engage my core and hold to do the movements.
Vey excited for the opportunity to compete and school on this horse!!!
I hope you got on well and learnt loads!