Thank you for your responses.
After further investigation, it is -ve, and part of the factory fitted alarm. Panic over!
It is a year 2000 Autohomes Windfall.
There was an excellent website someone set up for their Autohomes Wanderer: http://www.ourwanderer.uk
It was such a good site, with so many great examples of modifications the owner had made. It is such a shame it no longer exists.