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@bobble its quite hard to make out, but the older timber (not sure if joist or not) above the missing gyproc ceiling looks discoloured and there looks to be some white mould which would suggest there is a damp issue up there. Are you able to shine a torch up there and see what is above it/along it? Water will track across all the things to find the lowest possible point.
To add to all this speculation I think I would be looking at where your neighbours window meets your render and make sure there aren't any cracks, as well as the gutter above water may be getting in and finding its way down to your ceiling.
Might also be worth asking your neighbour if they have any damp issues on that area as well. (although I seem to recall he's not the most helpful of neighbours possibly?)
andos-
the timber that looks like a scaffold plank definitely looks discoloured yeah and there is a gap between that and the external wall, which is interesting.
I personally didn't see much with a torch. Not rained for over a week and been no issues. Planning to get my head up there when its next raining and get a camp chair on the neighbours drive to watch how the water runs..
bobble
More plasterboard fell away. Insurer can't do anything until a credible company states exactly what caused it. If it's a gradual thing that has happened for a long time, rather than related to a one off event, won't be covered I don't think