What features are you wanting?
Maplin do a cheap and cheerful one that does the trick and pretty easy to install.
...and it really is cheap and chearful!
Wired one into my old cinq, and it was a pain in the arse!
The Shark alarms from Maplin have good features for the money (shame they don't work very well!) - unfortunately I ended up turning motion sensativity, and volt sensing off, and just relying on door trigger sensors.
The motion sensor used to go off everytime a car with a loud exhaust drove past, and the volt sensor used to trigger if someone walked past on a mobile phone!
In total I spent about £100 (orginal alarm box, extra sensors, extra triggers), for a system which was a bitch to wire up correctly and discretely, and which only had limited use.
IMO it was a waste of money (ah the beauty of heinsight), when you concider that for £300 you could get a proper alarm professionally installed...
If you're gonna put an alarm in, get a basic door trigger one off ebay or one of the cheap cheap cheap systems from Maplin - don't waste too much money unless you're planning on blinging your car up to the 9's!