The standard loc8tor is £59.99 in Maplins including two fobs (you can get it a bit cheaper on ebay), and it's about £24.99 for two additional fobs. I found them really easy to program. Basically you have a little handheld unit and when you switch it on you can tell it which fob you want to look for and it gives you directional arrows to follow and it beeps more and more quickly the nearer you get to it. On a clear range it is supposed to work up to 600ft although I haven't yet determined how much this is hampered by fences, walls, shrubs etc.
There is also one called Loc8tor plus which is about £100, this includes all kinds of other spandangley gadgets which I don't think would necessarily be helpful locating a wandering cat.