Looks like they were just oppertunists mate as they wernt well prepared, Good job you had the alarm fitted though.
Alarms are great for the oppertunist theif as in your case, just scum having a go in a car park or somit. If they were professional theifs you need more than a normal alarm or clifford fitted. A Pro will get his target no matter what.
This is where G.P.S. Tracking comes into its own.
I have a "Tracker" and a catagory 2 thatcham alarm & immobiliser factory fitted to my Scooby.
This means that even if they do get past the alarm they then have the immobiliser to get past before they can start the car, even if they were wizz kids and got past that then the tracker would kick in notifying the police the vehicle is being stolen and the exact location being relayed to traffic vehicles in the area allowing them to "Track" the vehicle immediatly.
Costs a few bob but worth it in the long run i would say.
Also, if this happens the police recover your car, stay with your vehicle until a "Tracker" security guard attends your vehicle and then "Tracker" delivers your vehicle to any destination of your choice within the U.K. and the security Guard stays with your vehicle until it is safely delivered to your destination, Only then will the security guard leave your vehicle....
Now thats an alarm.....
However, i am gutted for you man, but looks like you got away lightly.