Looks like I am going to have to vent outside the roofspace by the sounds of the replies. Not overly keen to go through the brittle 1950's tiles so looks like it will be cutting holes for the vents through the asbestos eaves.
Wondering how the best way to cut say a 150mm dia hole x 2 through asbestos sheeting might be. Obviously a good dust mask. But what would be the best tool?, jigsaw, holesaw etc. Not really experienced in cutting this stuff.