If you want to know anything about
roof ventilator's or whirly bird's have a peek at this website.
There is a good app where you can see how a whirly bird would look on your roof with different colour schemes.
Also, there is a good visual aid that explains how they work in winter and in summer.
If the government ever gives out grants like the pink batts, it might be worth investigating even more.
The FAQ's page is good for some practical points about where to install them, and how much money they will save you.