Hi Kevin and Ed,
Really appreciate your input. After posting this afternoon I trawled the web looking for a wiring diagram, the link on Ukat for bmw wiring diagrams doesn't seem to work. I came across an article about a user enabled immobiliser fitted to some models of bmw. The user can input through the satnav a four digit code which can immobilise the vehicle. If the code is forgotten there is an emergency override procedure. The procedure is to disconnect battery for two minutes, reconnect, switch ignition to position one, alarm sounds for a short while, display on satnav also showed a timer counting down from 10 minutes . Also on the display is space to enter the code. If you have the code you can enter it at this point. After it has counted down to 0, car should start. This to my surprise worked !!!
I contacted the lady who owns the car to see if She was aware of this and She didn't know anything about it. She has had the car for six years. The car starts every time now and as I understand it the immobiliser only operates when the user activates it on leaving the car, it does not re arm. I have kept hold of the car to keep trying it . Sorry for the long winded post but it might save someone a bit of time in the future. Still don't know why it set itself though??