There should be two pins on the circuit board which I think are marked 'rst' if you place a screw driver across these and short the circuit by turning the power off and back on (I did mine by a wall fuse under the panel) it should perform a factory reset.
Gary
February 2010
Ring 0208 304 5740, they sorted mine.
Ronnie
April 2009
totally disconect the power and battery for 24 hours re-install now you will enter the engineers mode code 0000 or 1234 this will be your new code you can then change it to something you would prefer
karen johnson
March 2007
The manual for your alarm can be found at the following link