Just a quick update, so I don't spread any untruths...
This is the picture of the clock-spring connector I posted previously from the wiring manuals:
This is the connector I hadn't seen until today, which is shown right at the back of the PDF covering the MK2 wiring (2011/05 onwards):
So they did swap around the ground and signal wire for the cruise control buttons. Damn, only my first boards (that have no polarity requirement) will work on a MK1 unless I do some messy rewiring for the newer board. That extra wire I see in my harness is for the "Push to Talk" button as well, not that I've got the button - but I wonder if it would work if I did. Although the schematic says pin 1B is used by the airbag along with the 4 other pins, I don't think it really does as it makes no sense to source power through a relay coil?