Cheers m_c, I think you've got it right, the probe has a tiny circuit board inside with two LEDs (green open and red for a touch indication) so I think the power + wire is mainly to supply these so maybe the earth and signal hooked up to the Gecko pin is all I need. You won't believe what I have done this passed week to try a get this working, here's a video but don't laugh. I'll give it a try tomorrow.

https://youtu.be/Ph3E028JaNc