Okies, hopefully this will work and I haven't bought it before looking carefully enough!
I have a Kenwood KDC-6534U head unit which does have the changer control output
I now have a Kenwood KPA-SD100 control adapter 'thing'
It seems the SD100 is for 'older' Kenwood head units but given it fits with the changer control connector I am assuming it will work with my newer head unit and also with my Astra? (MK4 2002 SXi)
Don't want to go ripping the box open nor the car dash if anyone already knows this won't work? Possibly teach me to buy before checking if it isn't the right bit! Ideally I think I need the PC99-X06-DIS but fingers crossed the bit I have will work?