vauxhall vectra 1999 2.5 v6 starter motor not engaging?

Hi there. Ive just put a new battery into my 2.5V6 as the previous was knacked. Now the starter motor just wirs away and doesnt seem to enagage when you turn the ignition key. The starter motor will still try and turn the engine over when its in gear !! bizarre. It has a factory fitted car alarm, is it possible this is the cause, the alarm did go off a couple of times when I was changing the battery, and indeed carried on going even after the battery was disconnected. Thanks. pete