Jaguars are high-end luxury vehicles. Each model, including the XJ8, comes with a keyless entry feature that allows you use a remote to control the door locks and the boot release. If the keyless entry remote that came with your Jag no longer works, it might be due to a dead battery in the key fob. The key-fob battery is easy to change and does not require any professional technical assistance.
Purchase a new battery for your Jag's key fob. Contact an authorised Jaguar dealer or auto parts store and provide them with the year of your XJ8 so you can get the correct battery for your particular fob.
Pry off the back of your key fob with a small flathead screwdriver to access the battery compartment.
Remove the battery and replace it with the new one. Reattach the back of your key fob. Test the new battery by using the key.