Everyone,
Thought I'd mention how pleased I am with a cheap-as-chips garage alarm bought from my local chemist!!
It is just a wee battery-powered PIR sensor unit which sits/is mounted anywhere in the garage and, if alerted, activates a small battery-powered unit which can be put on chime or alarm anywhere in the house.
I'm sure you seen them about (cost about £30), I was genuinely surprised that it works well. I'm hoping to run a sensor from the house alarm into the garage when I get time, but as peace of mind goes in the meantime it doesn't get much better value than this!
Also, for those without a garage who are concerned that they mightn't hear their car alarm, the sensor unit can be left in the car overnight instead.