Bosch Lambda, 85 euros to your door
http://www.ebay.co.uk/itm/0258003477-BOSCH-OXYGEN-SENSOR-LS3477-LAMBDA-SENSOR-BRAND-NEW-GENUINE-PART-/151656991718?fits=Car+Make%3ABMW|Cars+Type%3A525i& hash=item234f760be6:g:EV0AAOSwNSxVNoZk