306 alarm problem

I have an N reg 306 xsi. A few months ago I had the alarm ecu replaced and since then each time I set the alarm it goes off every 5 mins. After my local dealership have had the car for a full day they think the problem is with the ultrasonic sensors. Disabling the sensors by pressing the button on the steering column stops the alarm from going off. Can anyone tell me how the sensors can be made less sensitive? Or how much it would cost to replace them? If the expense gets too much I'm looking at having a different alarm fitted and just turning the original one off. (I'm told that it could be a gap in one of the seals around the doors or windows triggering the sensors) Any suggestions on what I should have fitted? Has anyone had similar problems? Any help appreciated, I'm trying to find the easiest solution and am not fussed about using my local dealership to do it (have lost faith in them along the way).