Our cats now wear a tracker called loc8tor. They have a distance of 400 foot, but we found arthur earlier in the week in a building that has recently been built, he was hiding in their plant room, and the tracker managed to locate him.
i would recommend them.
Thanks Puffin and DebraG, I'll order one - our gorgeous re-homed stray is starting to venturing out and is getting used to his new area and although he probably does know his way home - I need reasurance that I can find him if he gets stuck somewhere. Looks like a great product.