Am too lazy to do this myself..  But here you go Daz, give this a whirl..

Could it be a bung magnet though..?

Stolen from the interweb, credit where credit is due..

Heres some info:Adjusting Eddy Sensor Boards.
Often eddy sensors can go out of adjustment and become less sensitive. This can cause the eddy sensor to not activate when a ball passes above it on the playfield. To adjust an eddy sensor do this:

* On the under the playfield eddy sensor control board, turn the potentiometer counter-clockwise until the LED just turns on.
* Now turn the potentiometer back clockwise until the LED just turns off.

That is all that is required to adjust the STNG/ToM/RS eddy sensor. To test the sensor, put the game into WPC diagnostic's first switch test. Then move a pinball over the playfield area where the eddy sensor is located. The switch should activate on display. Also from the bottom of the playfield, the eddy board LED should go ON as a ball passes in front of the eddy board's senssor (this can be seen anytime, the game does not need to be in switch test.)