There are two types of pressure switch, the one pictured above and the type that are built into the back of the water inlet housing, the technique for setting them are the same, although the housing type can be a little tricker to set up.
1) Turn off all taps.
2) Turn on isolation switch to operate pump. If the pump doesn't work turn the knob on the pressure switch clockwise until it does.
3) Open taps to clear air in system, turn taps off.
4) With the pump running and taps off, turn the knob on the pressure switch anticlockwise until the pressure switch "clicks" and the pump switches off.
5) Open the cold tap, the pump should come on within one second with a single click. If it doesn't, turn the knob on the pressure switch clockwise slightly until it does. Close the tap, the pump should switch off, try again.
6) Try the hot tap, it should come on within seven seconds, it doesn't turn the knob on the pressure switch clockwise slightly until it does. Close the tap, the pump should switch off within seven seconds, try again.
With a good pressure switch and using these steps above the pump should work when you want it and stop when you want it to.
If it doesn't there could be one or all of four problems
1)If the pressure switch keeps clicking turning the pump on and off, and you have noticed black bits coming out of the tap, the filter has broken, needs to be replaced and the pressure switch needs to be cleaned out.
2) The diaphragm has split in the pressure switch and the pressure switch needs to be replaced.
3)If it doesn’t work at all check the 12v supply to it.
4) Check the micro switch inside the pressure switch, (the most unlikely)