Alarm advice please!

I have just bought a renault scenic which has cenral locking, immobiliser but no alarm. Ideally because im a lazy so and so, i'd like to make the central locking remote but dont want to pay renaults prices to upgrade the system so was looking at buying an alarm with central locking interface. Any ideas as to which is a good one, cheaper the better. ? Also one which is easy to fit, the central locking can only be worked from the drivers door, dont knwo if that makes a difference but i've told you anyway. Thanks a million Pete