I've been driving and maintaining spitfires since 1978. Currently on my 8th one, and I love them :)
I've had all the variations of the rear suspension fixed, swing, rotoflex etc. and in every case I still remember the BL service tech's explanation. The toe-in of the rear suspension is responsible for keeping the car at a correct ride height and removing some of the load from the spring. It must be adjusted with the 'normal' weight of the passengers in the car. AND you must roll the car back and forward without aid of the engine to settle the suspension before starting.
The transverse leaf spring is the upper link of the suspension. The lower link is in two parts... The axle (or lower wishbone in the GT6 rotoflex) and the rear RADIUS ARM are the lower link.
The fixed spring will still allow a bit of flex across the spring due to the mounting system and the properties of metal. The swing spring will allow more flex across the spring. Therefore, unless there is a definite sag (over 1" in ride height) there is likely not a lot wrong with the spring.
The radius arm is the member which actually locates the rear wheel within the wheel arch, and if you look carefully, at the correct ride height, the radius arm is not parallel to the chassis of the car or the road. It is angled down.
This is what gives the rear suspension in the spitfire an active part in the handling of the car. As the spring compresses on the outside of a corner, the wheelbase on the inside gets slightly shorter. The outside wheel gets slightly longer. This allows the rear to become part of the steering system of the car. When driving in a straight line, bumps will be effective on both wheels due to the spring, and the lengthening and shortening of the wheelbase has no effect. The slight toe in will assist the spring and shocks in bringing the rear ride height to the 'normal' setting.
If the rear toe is not adjusted properly or evenly, or the busings in the rear suspension are worn, the handling and 'settling' are affected. In addition the tendency to 'sag' one side is enhanced. By having the rear toe adjusted and 'squared' the car will ride and sit evenly when driven and/or rolled in a straight ahead condition.
If, however, you normally pull into a curb (parking, etc) you will notice that the inside of the corner is usually lower than the outside. Doing the opposite will reverse the effect. And if the bushings in the radius arms, spring eyes, etc are worn, the effect is worse. This is because the suspension doesn't have enough rolling time to settle correctly. Additionally, if you back up (reverse), the ride height will actually increase slightly.
My current spitfire is a 1963 Mk I (original 1 owner) with the original spring. (Yes, I know the prior owner!!) It exhibited serious symptoms when I got the car. After replacing all of the suspension bushings (and many other things, not related to this) the car drives and sits squarely on its tyres. It handles exceptionally well up to the point of 'squealing tire hop' (another story), and comes to a rest in my driveway (20' long) straight and true. However, If I turn while coming to rest, the inside of the turn is always slightly lower than the outside.
A note of caution, however. I would suggest that you have this done at an alignment shop that works on VW bugs, because most of the other shops don't have a clue what '4 wheel alignment' is all about, nor will they attempt to correct the tracking of the rear wheels, which is what I believe most of you are experiencing.
Since I found out about this, it is one of the first things that I have done on any of my Spitfires, and I have never had a serious sag problem. Nor have I replaced any springs, except the one I broke when I hit a curb sliding sideways at speed...
Good luck to you all, and I hope that this will 'settle' the spring for some of you at least...