It might be the insurance company that has the keys as they generally request all keys when a vehicle is stolen. No idea what they do with them (lose them most likely...)Given it has no keys that means you'd need to spend £200 before you could start or drive it. Given that no test would be possible, how sure are you that it's 100%?
I'd ring the previous owner (from the V5) and see if they have the old spare key. Assuming the original was nicked when the car was pinched, there's no reason why the previous owner wouldn't still have the other and be willing to hand it to you.
Assuming you do get it sorted, get Merc to deactivate the other yey - after all, there's a scally walking around with a key for the car.