Hi Paul,
I'm sure the plug in system is made by CEC, also fairly sure they are still on the go so just wondering if you have got an older out of date phone number. A web search cam up with this link and number.
http://www.businessmagnet.co.uk/company/cecpluginsystemsltd-10388.htm
Have a look see if its a different phone number
Our old Bailey has an IDM-2 Plug in Systems alarm.
If that's the same one I'll get the instructions looked out for you.
You'll probabaly find that you'll need a new sealed battery if it is the same unit and has been left without being charge for more then a few weeks. Locate the black box, unscrew and drill out the 1/8th rivets with care, and replace the battery and then re rivet. Maplins keep them, Yuasa.
Surprise surprise all connection are Plug-In style, but the battery is sealed in the control box along with the alarm siren. Normally it's found somewhere under a bed or in the bottom of or behind a floor cupboard panel. The siren sounds through a hole in the floor. There is an under floor vent cover to prevent water spray entering on the Bailey so you should be able to locate it by checking underneath the van.