Vauxhall Vectra B 2.2 sri (2002) alarm sounding?

Hi ive just replaced the Central locking unit which lives behind the kick pannel on the drivers side (as the mrs smashed the only key fob we had).


When i took the connectors off the current unit the alarm went off (I guess I should have disconnected the battery before i started the job school boy error) I've put the new (second hand unit from the same year car and model) unit in and the doors lock and unlock fine with the new fob and the car sarts and drives.

Only thing is when you use the key fob the alarm sounds for about 1 min then turns off. It also does not turn off if i start the engine quickly??

Any ideas whay this is happening or more importantly how i rectify it??

Gaz