Hello Lucy, How old is the current battery? I ask since there maybe a current drain on it, rather than it just being well passed it’s best. If it’s over 5 year old, it would be wise to renew it anyway. What brand are you intending buying? Important, since these cars are certainly battery-efficiency sensitive. To ensure you don’t lose any codes then you would be wise to invest in a a cheap saver device, which acts to keep 12v to the battery cables when they are disconnected. Finally, you will need to code the new battery to the car, otherwise it could be seriously damaged by being overcharged. A forum search will come up with the advice you need. All in all, a DIY replacement may not be your best option in your case. Regards, Gareth.