The Inward Flow
If this house, from the 1820s through to the last decade, shows anything it's an excess of pretentiousness over sense.
Thus, rather than make the valley roof of the servant's wing drain away at the far end, oh, no, that would upset the appearance of the roof facing the road. No, we'll drain the water landing on 50 sq metres of roof down a 2" pipe in the middle of the house...(to where, we have absolutely no idea).
Hence, for most of the last 200 years, water has been dribbling through ceilings in the servants wing when it rains hard like it did today.
Thankfully, for the past two years, and today, it has run down both the original 2" pipe and a 4" pipe. One of our more comforting 'Dell House Fixes'.