Something is obviously faulty and causing a current draw.. I see you're in the UK, I believe you guys had Trimble traffic monitors that were notorious for doing exactly that. Phone modules can also go bad and draw current, and less commonly the nav units. Another common suspect is the final stage resistor for the climate control blower, they can actually cause the interior fan to run when the car is off and flatten the battery.

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03/2002 4.4i sports titan silver