HI DAVE THIS IS A POSSIBLE PROBLEM WITH THE CAS (IMMOBILISER UNIT). FIRST THING TO TRY WOULD BE THE SPARE KEY & IF FAULT STILL THE SAME, POSSIBLE PROGRAMMING UPDATE REQUIRED FOR CAS SYSTEM VIA BMW SSS SOFTWARE. IF SPARE KEY DOESN'T REMEDY THE PROBLEM. ADVISE TO TAKE TO MAIN DEALER AS AT THIS AGE, WOULD PROBABLY (IF NOT IN WARRANTY) GET GOODWILL FROM BMW GB. HOPE THIS HELPS