nissan interstar van wont start?

The van wont start and the red light that says the immobiliser is on flashes like mad, have tried sare key no luck, un-clamped the battery then re-clamped still no go, then it will start after hours then back to no start, can i disconnect the immobiliser?? has anyone else had this problem and can you tell me what to do thanks.. its a diesel van 53 plate. also had a new fob battery three months ago and the light still flashes but it wont open the central locking now. dud battery or big problem.