What tradesman do I need for damp under a bedroom window cill

Background info: End of terrace 1900 build house. I had the walls skimmed recently in the back bedroom but only after stripping the lining paper did I notice damp patches under the window. There seemed to be some remedial attempt to patch plaster bits of that area and it was these bits that were damp. The plasterer (who also deals with damp) took parts around there back to the brick but the brick was dry and actually needed PVA for the plaster. Since it was skimmed there was a small patch of damp still. The exterior sandstone cill it a bit battered and it was silicone sealed at the top where it meets the window frame as there was a gap. Independently I have had 2 roofers check the roof and both have given the ok there. One said the pointing is fine around the window as well. The edges of the window frame seal could do with re sealing externally as well I think so this may be something to do with it. There is also an air brick above the window on the same side as the leak. as the vent has not yet been added internally I can see right to the other side and the 2 layers of bricks are on show with the cavity gap also on show. There is bits of wood to form a makeshift "tunnel" between the internal and external sides but there is the cavity gap exposed as well. As I have had a plasterer who "deals" with damp and also 2 roofers not be able to remedy the problem I am unsure as to what tradesman I need. Any guidance appreciated! Andy