Hi Gordon,
I think a stone-built signal box would be a bit unlikely in such a situation. Brick is fine, but surely if the sighting requirements needed a tall box, that is what would have been built -- a tall box all in brick set in close against the stone wall. That stone plinth looks wrong, especially in similar age/colouring to the walls and with the steps at the wrong end for the box. At the time the original stone walls were built the signalling would have consisted of a man on a shaky wooden platform waving a flag. Centralized signal boxes with interlocked lever frames came later, and wouldn't have been designed into the original stoneworks.
If you leave the platform as-is, at the very least it will need some heavy modification to allow the rodding runs down the vertical face, and colouring to indicate a later construction.
regards,
Martin.