Thanks for your reply Matt. I changed the roof to roof faces and used a polygon with the clipping tool to create an opening. To delete it, the only way i could was to use a rectangle and add surface which effectively filled it in. It was a bit more difficult to select it to modify it.
While i was creating openings in my roof for 4 gable roofs it somehow deleted part of the main roof. Not sure why it would do this! Since then, I've had graphic trouble in OpenGL mode while in 3D orbit. Very pixelated and walls and roofs can sometimes disappear or go transparent but not deleted from the file etc. Worried it's now corrupt!