Open a tap, so that you have a steady fow with no spluttering.Turn the pressure switch fully clockwise-the pump will continue to run. Close the tap. Turn the pressure switch knob anti-clockwise steadily until there is a click. Continue a turther 1/2 turn. This setting should be about right. The cold side should shut the pump off quickly, but allow longer say 10 secs for the hot side. A minor adjustment may still be required.
If you switch from mains to battery power (and vice-versa), expect to have repeat this process
I hope your switch is easily accessible-I moved mine for this reason