Just to close this thread off, and in case anyone needs similar information, this is the reply I received from PCL:-
"Sorry the MK3 cannot be adjusted or recalibrated.
The gauge should be oiled, this will prevent the gauge from sticking, which can give incorrect readings.
It should be oiled a few times a month depending on usage. The oil screw is located above the serial number."
Well, mine has never been oiled since I bought it, so hopefully this will help.
Rob