How can I fix Daimler 1990 XJ40 alarm; goes off at random?

How do I repair/stop alarm on a Daimler XJ40 (1990) going off at random. At the moment starting the engine and letting it run for about 10 mins ensures it doesn't go off for rest of day other than when starting. It goes off for a few seconds when I start the car.
Dave Johnstone, January 2009


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