I worked out a solution for this:
1. Centre punch the remnants of the spring & pin assemble. This will cause it to drop down into its hole in the ally piston body.
2. Get a 4mm allen head grub screw and matching thread tap.
3. Tap a thread into the steel caliper hole to accept the grub screw.
4. Bit of copper-slip on the thread and screw in the grub screw once the threaded hole and the locating hole are lined up.
My calipers are now rock solid and ready for refitting to the car.
This to me seems a far simpler method than that used by Lancia originally, usual over complication of design I guess ;-)
sounds like a good job done, to be fair to Lancia I think the original set up is probably designed more for manufacturing convenience than serviceability no doubt much quicker to assemble in the factory