Thats the likely cause.

Building regs state the level of ventilation required (Although Part F covers home ventilations, roof ventilation is covered in Part C Note; the scottis spec is pretty much the same except the doc content is pretty useless!).

http://www.planningportal.gov.uk/uploads/br/BR_PDFs_ADC_2004.pdf

Given the house age, this is likely to be in the form of eaves or soffit vents and there are 2 common reasons why the ventilation is compromised.

1) Insulation has been installed tight up against the felt which has restricted the air flow. various things are available to sort this e.g.:

http://www.harcon.co.uk/eaves/index.aspx

Note: although idealy installed during construction, these can be pushed into place from inside.

2) Plastic facias have been fitted and no vents included.

Again, easy to overcome by fitting vents to the soffit part of the facias.

Another option is to add some air bricks to the gable ends (assuming you have them )