NOTE This FAILED!!!!
LJC18A3-B-Z/AY PNP NC 3-Wires 10mm Capacitive Proximity Sensor Switch DC 6-36V
a) Seemed to give different distances depending whether it was on the right hand side of the Print bed of the left hand side of the bed.
b) The LED would flicker some times as it moved from On to Off sensing. On a 3D printer I thing this could cause a problem because when Homing the head touches down the goes back up then back down as a double check. If the On/Off flicker is happening during this time false signals could occur, some video abut this flicker:-