My Doro 612 keeps switching itself off. It is not low on battery and there does not seem to be any pattern. I can do it twice in a day or not for a week. Left on it's own or taken from bag and checked. The back cover is not loose, the battery itself is not loose. Any ideas please?

Check if there's enough battery power and if the battery is properly inserted. What were you doing with the phone before this happened?