I think I have finally solved my location problem, I contacted a company called FollowGB who supply a range of tracking software and hardware as well as an online control panel for locating things and people.
At first glance all there products seem tied to there own tracking system but they have said that they can reprogram the devices to send data to any server and seem quite happy to do this for me.

The device they recommend is called Trim Pro and is a small self contained GPS and GSM system which runs from 4 AA batteries. It costs £235 so its not particularly cheap and is at the higher end of my budget but it does seem ideal.
I am still waiting for the company to get back to me with some specifics but all going well I should get this within the next week or two, when I do I will post a more detailed review.