I've had a similar issue and I suggested getting a new battery. The dealership firmly told me that I did not need a new battery and that the problem lay with the car being used typically for short journeys. I got it booked in for a new health check later that week and I was told to take the car on a run of 30 or so miles along the motorway and back beforehand. The car passed without any problem. Ever since I always take my car on a decent run before it goes in for a service and have had no problems since.