Rob
Keeping a supply to the car whilst changing the battery saves a mass of messing around. Sometimes referred to as a "Tender". I have my S Type set up with my CTEX MXS 7.0 so as I can keep a supply to the car whilst removing the terminals from the battery. If you do not have, or do not want to invest in buying equipment to do this I feel sure that certain battery suppliers will have a means of doing this whilst they fit a new battery for you. I am guessing but I would suggest the likes of Halfords or similar would have the necessary bit of kit to do this job. It has surely got to be worth a telephone call to them.
Good luck with the job . Best Wishes and Regards, John