Oops, just had one of those "blond moments"
I had the whole outboard casing apart looking for this plug and assumed the drawing showing the comms port was the grey plug. This is the Joint connector which explains the shorting cap!
The rubber covered plug looked like a engine warning buzzer to me so I didn't try and take the cover off or look at it closely as I didn't realise thats what it was.
As soon as you said above the ECU it was just "Do'h!"
I assume I can have the trim analogue gauge working as well as the network version? Its one or the t'other?
Now my credibility destroyed I'm away to sit in the corner..............