Somewhere in my "box of bits" I've got a fairly normal timer which used to be on the immersion heater in the old house. It had a button on it which you could press once for 1 hour on, twice for 2 hours on, or again to revert to whatever state it was in before you pressed the button. Dead useful for turning it on when you didn't normally need it, and it didn't interfere with the normal times.