Nigel,
I've taken some photos of my springs and think i can see the issue. The first is with the car off the ground and the spring at its fullest extension. you can see the damage on the coil's powder coat so the springs must be binding together. There's no feasible way for the mounts ever to come in contact (hence lack of damage)
With the car back on the ground you can see the coils are very close so it's inevitable that they will come into contact.
So I have to decide whether to go with longer springs as you did or just go for a higher spring rate. The current ones are 400lb. I seem to remember you saying you tried 600lb and they were too hard? maybe 500lb with 7" would be the compromise as I actually find the back end too soft ( I lift out of my seat going over cattle grids fast, which bearing in mind I weigh over 17 stone...)