110v heater from one leg of 220v well pump supply

A bit tricky Treb. If that third terminal is truly a return then yes. However it might prove to be an actual "ground" in which case using it as a return could be risky

Someone more familiar with modern wiring practices will doubtless respond, letting me off the hook. Presumably there's a socket, so wondering whether 3 terminals or 4

No socket - three wires direct to pressure switch from panel in house. ( two 20A breakers @ panel in house). I can't access the pressure switch right now (Damn snow is 18+ inches deep around the well) so I don't know exactly what the wires are - I do know it is a 3 wire setup. Thanks for your input

Not at all Treb, glad I could help

It happens so rarely

I don't think its a good idea to use one leg of pump supply to water heater, because if there is any problem in pump then it may also lead to flow current in the water heater. It could be dangerous.