W210 E220 alarm problem

[jgannon]

The alarm on my E220 estate (W210) comes on when I lock the car using the button on the electronic key. When I press the button on the key all the doors lock and the indicators flash 3 times as normal. After about 15/20 seconds (but sometimes longer) the alarm goes off and when I press the buttons on the key to open the car and relock it the alarm tends to go off again immediately. I have disabled the interior motion sensor and the tow-away protection as detailed in the manual but the problem remains. I have opened and closed all doors, boot lid and bonnet and set the alarm each time but the problem remains. The only way I can lock the car is to lock all the doors from the inside (apart from the drivers) and use the mechanical key to lock the driver’s door and boot from the outside.