Originally Posted by st220kyle

How strange, not on my Mondeo but on our old Sport Ka we had i had exactly the same fault. The lights flashed and never made a noise but the siren was definitely not disconnected.
cut a long story short, it was the switch under the bonnet which you say you've already replaced.it was all wet,the connections had gone all green and shity. changed that and problem solved. Might be worth you double checking yours?