I believe it was the room full of old papers on one of the upper floors of the main mill that was set ablaze.
From what ive seen from the exterior, the damage doesn't seem too extensive as the fire brigade had it under control pretty fast.
I went down there on the night of the fire as i live in the area and i could see firefighters walking inside on the floors so the damage can't be too bad.
I think the fire was just in that one room i couldnt see any smoke damage on any other parts of the building except those few windows
This was taken from the main road at the gate (i apologise for the quality i just took this as i was walking past)
This picture was taken from the lakeside
You can see faint smoke damage coming from the middle set of windows but nothing too extreme
Hope this helps