ssshhh, dont say that, its not the advice thats wanted !!
I think most of the responses have been helpful actually Jemma. You choose what tips to take on board or not as the case may be. But nowhere in your post did it give any info as to what experience you have with shooting horses and also I took it from your OP that you are not a horswoman - am i wrong ? Being a horsewoman myself (admittedly not a polo player) I gave you my opinion based on the type of reactions I deal with regularly, i.e. spooking at a leaf, having a fit at a different shade of grass, etc etc and a solution to a problem.
Have fun at the polo match