Start it without immobiliser first? You mean your key will not dis-arm the immobiliser? If so then its new key time, you have checked the battery in the key?
If its the Mk1 or Mk2 Megane you have to open the car with the plipper key, and then you get 15 sec in which to start the car, if I recall.
Could well be new key time, I fear.
John
Probably cheaper to get immobiliser disabled on older cars, cost around £100 I think? and not difficult do do on meagannes.
Try this link http://ecurepair.co.uk/renault.php