My key won’t unlock the car, is the battery in the key fob low? [450]

It could be. It’s more likely that the key has fully discharged, lost its coding and will have to be re-coded along with the car and any spare keys you might have. Try to use both keys equally and often to prevent losing the coding. There has been no satisfactory explanation as to why they do this. If the battery is low the indicators will flash 9 times quickly (indicating you can use it 100 times more – so change the battery asap). When changing batteries observe correct polarity and change them promptly. Batteries are available in many retail outlets and garages. Battery size is CR1225 (except for the City Coupe 600cc, which has a 1 button key and takes a CR2016). See the handbook for how to replace the battery.