Sounds like the infamous alarm spider unit to me?
If so, LR offer a bypass plug to replace this module at the expense of losing the immobiliser, or alternatively you can open the module up and try drying it out before reinstalling. Check for dry solder joints at the same time as these units are prone to causing the engine to cut out and refuse to start as they age. If you search something like "Discovery alarm spider" on Google then you should find a thread on another forum with precise instructions on how to get at it and the part number for the bypass plug.
