Welcome to the Club, Matthew. A good battery of the correct size is essential for Jaguars, as there are a lot of electrical bits that need power. On the Jaguar, you will find that if a battery is failing and not holding a charge the less essential functions of the car close down, and I guess that warning lights are amongst those. When you gave the car a run you were definitely in limp home mode, and an OBD2 reader should tell you why. Let us know how you get on. The s type is a great car and the 3 litre petrol engine is a unit well used by Jaguar. Regards, Peter.