I suppose I could try taking one of the upper panes out from the inside, and see whether I could access the soffits myself. Perhaps remove one or two screws carefully at first, lest they come out in a shower of rotten wood dust and beetles.
I can't tell from the photo quite what is going on but I would have thought you could splice a repair in there with Repair care. It's worth looking at some of their videos and maybe track down someone local who has done their course.
I'm not completely sure myself what is going on, but if I can get any closer access from the inside I should be able to gather more photos and perhaps sketch out the situation.
I suppose I could try taking one of the upper panes out from the inside, and see whether I could access the soffits myself. Perhaps remove one or two screws carefully at first, lest they come out in a shower of rotten wood dust and beetles.