Hi theolgcodger and welcome to the forum.
Went a step further after looking at then grid as i could not see the part number you specified there. Put your part no in Rimmers search box and came up with this,
http://www.rimmerbros.co.uk/Item--i-XQE000580
Unfortunately no longer available. The only thing i can suggest is that you phone round some specialist MG Rover breakers. Found this thread from 'tother side', a list of MG Rover breakers but it is from a few years back. Some may still be there though.
http://forums.mg-rover.org/showthread.php?t=154877
The other thing i have thought about is that maybe a local in car entertainment centre may be able to make one up for you if they have the right connectors
Hope this may help you,
Regards,
Dave.