The main board has had to be replaced, so Canon inform me.(posted 8356 days ago)The problem was this.
The body and lens stopped communicating. On depressing the shutter in lens AF mode the lens would not focus. Furthermore the eyepiece would darken as if the mirror had lifted but the shutter would not open. A further depression of the release would return the camera to status quo. The 'bc' indicator would illuminate which initially suggested that the camera was short on battery power. I should say that the camera would operate in lens MF mode. This had happened to me before usually in cold/damp conditions but not outside the cameras operating envelope. Usually 10 minutes in the camera bag was enough to 'warm-up' the camera and it would be okay. I had seen posts on the Canon FAQ at this site before, but Canon had always appeared reluctant to admit anything was wrong. I wouldn't class myself as a conspiracy theorist but i wonder that now the camera is out of production if they are prepared to admit that there is a design fault?
Andrew