I hope this didn't arrive too late. If the trial is like the NACSW trials in the US, there are 4 elements: Interior, generally a large room like a classroom, Exterior, generally like an outside courtyard or picnic area. A vehicle search of 4 vehicles parked in a row or side by side, and finally a container search which is boxes, suitcases, backpacks, etc. strewn about a large room. Your proposed selection of lens should work well for you. You will likely be asked to keep a minimum distance from the dogs so they can work without you as a distraction. Outside the telephoto will be ideal. Inside, you may want to stay with the 70-200 depending on the room size. The teams should be working methodically checking every nook and cranny for the scent so you won't need extremely fast shutter speeds. I doubt flash will be allowed as it will be distracting to the dogs. The 50 may be handy in a smaller room. If they will tell you where the hide is before each run (be sure to ask), you can position yourself for best shots as the teams hone in on the hide. Good luck!