Phil,
I think you need to be careful about the cost of a tracker vs. the potential benefits. I recently spoke to Ted Holt - the caravan clubs technical chairman. Ted advised several issues around the trackers that concerned me greatly.
The Police will only try to recover stolen caravans whilst in transit. Often the theives will take a stolen caravan a short distance to a known empty field and leave it for a week, to allow it to 'cool down'. This means that the window for sucessful use of the tracker may be limited to under one hour.
Part of the reason for this is that the Police wish to catch the thieves in the act, not recover stolen property.
The Police will not enter a travellers site, even if there are trackers activated -it is too risky for the officers and unlikely to lead to an arest.
Part of the Police's lack of desire to recover stolen caravans is because the insurance companies advise that 90% of those owners who do have caravans recovered, do not want them back.
In short, prevention is far better than cure, so your money may be beter spent on additional physical security.
Sorry to be such a kill-joy, but these matters of known Police policy do not seem to be advertised.