Update on problem! First, the vehicle was not dealer purchased, it is 1997 model. Left overnight and beep still exists and vehicle starts with no problem without activating the immobiliser defeat. So it seems to be an immobiliser problem. A bit more info as to when it started, I stopped at a filling station and turned off the engine, on returning to the vehicle I offered the immobiliser key to the sensor and instead of the normal beep it sounded like a relay chattering. This chattering is occurring every time I use the immobiliser key but the vehicle starts OK