Weird update on this - just had the house insurance surveyor come round as we were hoping to try and go down the storm damage route.
He's emphatically told us we don't need a new roof. Just take the old insulation out, fit the right amount in so it can breathe, and sort out a bit of leadwork where one house leads to another. It should therefore, be a much more palatable bill. I have had two quotes from contractors that the letting agents use, who both want 10k+. Seems absolutely shifty to me.