As some one who used to instruct at Shobdon (early 80's) I must say that I used to enjoy the intrusions both pre booked and impromptu. I never felt that there was any risk of collision with the lost ones as they were always well below me and never seemed to be going east west but always north south. The only time I did get a little worried was when I was instructing forced landings away from the airfield. But even then on the two occaisions I came a bit close to some thing green and grey and going very fast, I was not quite as low as they were.
( I did get my "revenge" on a tornando in the Firth of Forth once, but that is another (sea) story.)