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 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.