There are loads on ebay, you might find it easier to buy one with an alarm included allready, then you know for sure it will work, rather than trying to make the locking compatable with your factory setup.
I was looking into getting central locking last summer and in the end opted not to bother with an alarm. Ive gotta say it was pretty easy to install, beats going round each door any day.
I had al mine fitted at the same time...the guy who did it even came out to my house. Its a Microscan alarm and central locking kit i think was from demon tweeks. All works brilliantly apart from the rear door lock plays up sometimes. Where in the UK are you... can pm you his number if you like?