The gap had once been filled with cement, the failure of which let in a great deal of water which inevitably soaked in to the tower walls or ran down the inside of the building.
When restoring the tower we filled the gap with builders silicone which worked for a while but that too failed as it hardened and could not cope with the movement between the iron and the stone.
We needed a sealant which could bridge wide gaps and had much more elasticity than silicone. Enquiries led us to ArboMeric MP20 - a one part high modulus modified polymer sealant which cures on exposure to moisture vapour to form a tough but elastic rubber.
Super gloop, it seems to have done the trick.