it's basically orienteering on horseback - its mainly for pleasure riders so not serious competition. I'm hoping to try it this year (if foot and mouth clears up!). I think there are 3 stages - orienteering (basically a cross country hack), jumping and control of the horse (could be wrong!). If you go to the bhs website -
http://www.bhs.org.uk they have a section that tell's you all about it and where in the UK Trecs will be held