Hi jonathan, I have a mobile phone tracking device fitted in my caravan on the orange network. I use www.followus.co.uk to keep track of my van, it's not brilliant as it depends on where the mobile masts are located ie, if you lived in London you would most likely get a location fix within 50 meters. where I live in south Wales my caravan is in storage and I get a radius fix of about 1.5 miles, so the van could be anywhere within that 1.5 mile radius.
The good thing about mine is if the caravan was ever taken from storage the fix would not be the same as normal so I would know it had been moved.
I think the best tracker on the market at the moment has got to be the phantom tracker: http://www.phantom.uk.net/caravan/default.aspx the only problem is it's a bit expensive.
Hope this helps
Graham (Ponty)