Tracker Stickers
First line of defence in stopping a thief is to convince them that the vehicle is tracked - Even if its not. You can buy stickers on ebay that look exactly like the proper Tracker stickers for about £3 per set. Just search on tracker sticker on ebay.
GPS
GPS systems are very accurate but tend to be more expensive. A company called http://www.trackershack.co.uk sells a mobile GPS tracker unit for £234 which is tracked via a webpage http://www.followgb.co.uk. I've got these running in two vehicles at the moment and they really do work.
GSM
GSM trackers operate through the mobile phone network. They are not as accurate as GPS trackers but are very cheap to set up. Simply take an old mobile phone with some credit on it. Register the phone at http://www.followus.co.uk and thats it - You can track the phone to within about 400 yds. It costs about £1.00 to set the system up and about 20p per track. I've got an old mobile hidden away in my van just in case.
I also use this method to track my kids mobile phones so I know roughly where they are.
I hope this is useful to someone and if it prevents one theft it'll be worth it.
