Welcome.
The condition you describe is normal.
Spitfires have a form of independent rear suspension called "swing axles" (just like VW bugs). When the car is jacked up, the wheels will have "positive camber". When the car is on the ground, the wheels usually have a little bit of "negative camber". If your car has the original rear spring, it made be somewhat weak, allowing the car to have a bit too much negative camber. By the way, Spitfires have a single transverse leaf-spring in the rear. A weak spring on a Spit is not ususual and it's not a problem that I would fix right away (unless you see excessively loose parts in the rear suspension or the car is bottoming out all the time).
While you're looking, you may also want to check for any excess rust where the spring is mounted to the frame.