OK - got my kit, done the physical install bit, the changer accepts/ejects the cartridge; but the head unit does not recognise it...
I've asked a couple of independent specialists (including PCS in Milton Keynes) and they all say I need to go to a main dealer for get the retrofit s/w reprogramming done!
A minimum of half an hour of main dealer's labour charge plus the £100 I paid on eBay - i may as well as bought it off a main dealer in the first place!
I'm not giving up on independents yet...