What type of tradesman do I need?

I live in a listed stone building with wooden sash windows. Above three of the windows is a shallow semi-circlar gap filled with lime mortar (the same gap on my other windows appear to be filled with timber). I'm not sure what the mortar is adhering to or what sits behind it in the gap. The mortar is cracking and letting in water. What sort of tradesman do I need to advise me on options for addressing the problem? Would it be a renderer, or a joiner (since it sits above the window and some of the other windows have timber in the gap), or someone else?