Thanks @twopenny the windows are timber and single glazed and very old. I suspect water ingress and the external concrete sill underneath the window is bending downward at the left and right side so I think rainwater maybe running downwards and into tiny cracks. The seller hasn't lived there for over a year. And there appears to be only one subfloor airbrick under mentioned window but nothing at the rear of the property for flowing through ventilation. Not seen in the loft but got photographs from the surveyor and he did say the timber looked fine. Sorry to hear about your ceiling – can imagine very frustrating!