I have a 1999 v reg clio and it sounds like i had the same problem my immobilser kicked in and i couldnt start my car called round 10 different places for help & no one could tell me what was wrong kept telling me to bring the car down, i think this was a way of getting money from me. But i checked the net and found something that may help
STEP 1 : TURN THE IGNITION ON ON THE CAR AND LEAVE IT ON
STEP 2: NOW REMOVE THE 2 LEADS OFF THE BETTERY & LEAVE FOR ABOUT 5 MINUTES LEAVING THE KEY IN THE IGNITION
STEP 3 : RE-CONNECT THE 2 BATTERY AGAIN, THEN TIGHTEN
STEP 4 : TRY TO START YOUR CAR AGAIN.
This worked for me first time i hope it works for you
paul fulton, April 2010
