I dont think you can have a remote added yourself as each one is coded to a specific alarm, so if you buy it off ebay, you will still have to have a guy come out and code it for you, thats where they make their money.
edit: check if you have a reciept with the alarm, because when i bought mine for my other car i opted for the free replacement and service of my alarm if anything went wrong, including lost key fobs, think it cost me an extra £50 but i thought it was worth it, should tell you if you have it on the reciept, could save you a few quid.