A silly question I know, but is your loft insulated?. If it is, is there a space between the insulation and the outside wall to let the air curculate through the loft space. The insulation should just go to the edge of the brickwork, you should be able to see daylight Cutting holes in the in the underfelt won't help, the felt is there in case water gets under the tiles. No extration fans should vent into the loft space, if there is re-route it to the outside.
Homer., February 2010