I walked over to the car dealership today, to hand over the spare key. They need both of them for what they're about to do. They're going to have to replace one of the on-board computers, something called the BSI ("body systems integration") unit, and then teach both keys how to talk to it. It's not a part they usually stock so they're waiting for it to arrive from distant parts. It'll be about £365 (S+F), and they expect to have it done by the end of next week.