If you had money to spare I'd say get a tracker as well, but if I had the choice again I'd put it into £600 of Almax chains and concreted in cround anchors...a tracker "could" help but I am now a firm believer that physical security beats all...
In fact I'd say, put the £600 towards one of those bike sheds - dunno if it's on this thread but I'm moving to another place with a garage now, and getting one of these bolted to the floor inside it:
http://www.motoeurope.com/secure_a_bike.html#Racer
Expensive, but if they have the ability to move over half a ton (the unit weighs 350kg for the 'Racer'), which is bolted to the floor, I don't know what else can be done!
Sorry to hear about your mates...having just the one nicked is so sickening, I can't imagine what it must be like to have 3 nicked!