The Continental GT, GTC and Flying Spur built up to 2017 carry two batteries in the boot. The one on the left powers the car's systems. The one on the right is kept for starting. They age at different rates, and the left one usually goes first because it feeds everything while the car is parked. Test both together. Replacing one tired battery next to another tired battery only postpones the second call.
Bentleys are rarely daily cars, and that is the problem. The alarm, tracker and control units draw power around the clock, so two or three weeks in a tower basement can drain the systems battery. Bentley supplies a battery conditioner with many of its cars for this reason, but most JBR parking bays have no socket nearby. After a replacement the new battery should be registered with a diagnostic tool, and a memory saver keeps seat, radio and suspension settings in place.