Craig,
There are a number of tracker systms on the market but the one which seems to get a number of favourable reports is the Phantom.
I spoke to a police officer who was manning the caravan security stand at the Earls Court show and he was very enthusiastic about it. He is a member of the committee who advises the industry on security and has had one fitted onto his own van.
I have just ordered one for my new van (it is being fitted today) and will know more after we pick the van up at the weekend.
Now the downside. They are not cheap. The Phantom Pro-active, which will send a message to the call centre as soon as the van is tampered with, is