OkayBloody hell! Our last house was built in 1949 so same roof structure as yours but the space between roof trusses was around 800mm and we didn't have any issues like you are having...
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So you don't have dust in the roof cavity?
The only option I can think of is to remove the tiles, replace the existing battens with bigger battens to accommodate the span, put a roof plastic and put the roof tiles back.
Has anyone been through something like this?