Damp Problem on Sloping Ceiling

[tra98sew]

Our house is 1900s brick detached 2-storey house which is 'L-shaped', with pitched roofs on both sections of the 'L' then a join in the middle. The back part of the house has lower ceiling heights and the edges are sloping under the eaves of the roof.

We have recently had additional loft insulation installed and the landing re-plastered but noticed this morning that there is a water mark near the corner of the landing ceiling (actually the sloping section which is under the eaves). There was some damp damage in this area before we moved in, but the area has always been dry since we've lived there.

I am wondering whether it may be caused by the loft insulation being packed down into the eaves and reducing ventilation, or if is going to be coming from somwhere else (maybe the flashing on the joint of the actual roof?), but either way would appreciate any advice or solutions anyone can offer.

Thanks