This is what me and my brother have got:

http://www.directcarparts.co.uk/buy-car-accessories/toad-ai606-interactive-alarm-thatcham-alarm-system-fitted-p-13336.html

Best alarm by far, I know its pricey and up market but, it all depends what your willing to pay, for how good it is. The installation lad who comes around is spot on, pleasant, polite , will ensure every feature is working and then demonstrates it for you. He will set it up exactly how you want it, top nortch company

A laserline 996 V3 is all you need, is cat 1 approved and is a little over £100

plus - theyre piss easy to fit

Do you know a good place to get one of those alarms?

Gav, that alarm is good but it is a lot of money...

you have to splice into the ignition lock, run wires for the detectors, put the ECU somewhere appropriate (And earthed properly), run wires to the sounder (I suggest well hidden as you can unplug them and put them in water to disable), the central locking ECU if you have one, and you then immobilise either your ignition circuit or ECU and fuel pump. Total time - about 3 hours, or 4 hours super tidy

NI is a bit far to come to get an alarm fitted!

The immobiliser on that would be better, but you could use both I suppose