Thanks to both of you. Neil is correct, I have a Zumo 660.
Holding the clock doesn't do anything. When I release it I get the time menu.
I can get the engineering menu alright, but as soon as I open the waterproof door it switches back to "Close the door". I have once or twice in the past succeeded booting it into normal mode with the door open, but I think a firmware update has disabled that. At least I can't get there any more.