.........I thought that might be the case...............there were hardly any requirements in 70s building regulations for roof ventilation. You can imagine all the moisture from the house (cooking washing breathing etc) rising into the roof space. Particularly at this time of year, this condenses easily on a freezing cold roof covering.
You need "cross ventilation" from front eaves to rear eaves. ie from the flat bit (soffit) just behind the fascia board to the same on the rear of the house.
You can buy, for instance, small round plastic vents to install into the soffits (simple job as long as the soffits are wide enough).
Ask at the counter of a good builder's merchants - or check this site out :
http://www.screwfix.com/app/sfd/cat/home-main. jsp
...wish I could draw you a sketch..........would explain a lot easier than I can!