Hi all,
Sorry for not replying earlier, garage wanted to see the car when the dashboard light was on, so it stayed off for three weeks! Today when it came on I went straight to the garage. Solution seems to be that the brake light "holder" was faulty, loose enough to, for a split second, "disconnect" the brake light and tell the dashboard of the failure, but then go back into position so when you checked, the brake light was working.

Thanks for all your suggestions.