Thanks for your help. We did eventually manage to do it - thankfully! Though we had a heck of a job getting the connector off the back of the radio!!
The cable that plugs into the CD multichanger is round with 13 pins. The end that plugs into the radio is oblong and there seems to be 5 metal connectors along one side and 4 opposite. We gouged the plastic a little trying to get it out and broke a little hook off one side (it's there to help remove the plug but just snapped off when we tried!) but apart from that it's fine.