Hullo Luke
I, too, have a Dufour 40 (2004).
I can’t answer your question directly but, I’ve noticed that I have to move my finger around the push button to find a ‘sweet spot’ before the gauge needle moves on tank 1.
So it may be as simple as replacing the push button. You could by-pass the switch and run a cable across the gauge to see if it moves.
Failing that, if you’re in the UK, Force Four Chandlery may help. They supply water tank gauges that look very similar to those on the Dufour 40. It is an Italian make, “Osculati”
That’s about as far as I’ve got. But you could investigate from there.
Good luck. I’d be interested tio learn how you get on.