Update

[PercyUK]

Ok so the springs are the same length from the centres. I used a laser to check the frame and it looks ok to the untrained eye. I loosened of the axle and tried to move it the few MM that is available where the spigot from the top of  the spring fits into the axle mount. By pushing one side back as far as it goes and the other forward the combined movement of about 6mm seems to have made a lot of difference. I also loosened off the U bolts and encouraged everything to site straight as I suspected that one spring was pulling slightly encouraging the axle to sit further to one side of the jeep than the other.
I am feeling a little more confident and once the tracking is done will report back....hopefully with some good news. !!

[PercyUK]

OK, further update. I have borrowed a laser measure and I am now sad to admit that the frame is bent after all.
The clue was in a front bumper repair on the drivers side. I suspect that it hit something on that corner some time ago and has shoved the drivers side of the frame rearward about 2 to 3cm....  1" to 1 1/2" . This accounts for the rear axle being canted.
Unfortunately this means taking the body off. !!
On the plus side I can address the rusty floor whilst the tub is off but it doesn't make me feel very much better.
Having been around old cars all my life I am a bit disappointed that I didn't notice it when I first saw it.
I will let you know what I find in a few weeks time.