They'll charge for it - probably worth giving them a call to enquire first, but I have a feeling it's a ridiculous number of hours of labour (like 6 or so). If the warranty is expired, you might want to consider using an independent; the latest third-party diagnostic equipment is able to update the car's software and BMW are obliged to supply them with the updates. Birds BMW in Iver are probably not far from you:
http://www.birdsauto.com/