How can I stop the alarm on a Renault Megane going off after battery change?

A while ago our Megane had run it's battery flat, so I replaced it. When I put the new battery on, the alarm started going off and nothing I di would stop it. I have since changed the battery in the keyfob, just in case but the situation is still the same. I can lock and unlock the car on the fob - I can even start it, but the alarm keeps going off. I have unplugged the siren, but obviously the hazards flash every 20 seconds or so when the battery is connected. I phoned the autoelectrics blaoke who would say anything other than 'it's 150 quid mate' and then phoned back to say 'sorry, I meant its 85 quid mate' - it's not the immobiliser - it's the alarm. I don't want to go to a dealer because I know all they'll do is replace it and charge me accordingly. Can someone help me please????? Thanks!

Renault Megane Ragtop 1.6e on a 'T' plate