Luckily it was inspected in time and I was able to take it down before it fell. Just as well because it is 4m high above a stable entrance arch.
The two halves were pinned together and jointed using lime. The gnomon (metal style) was restored and repainted by specialists, I cleaned the face of the plate, and discovered and highlighted the markings with non-etching paint. It was replaced using stainless steel fixings.
The white stain is lime and stone from the repair. In a few years this will become less chalky and will no longer be noticeable