Thanks for the reply from Utrect... Sjonnie...
I have since bought off ebay the following items which should enable me to fit my Head Unit :~
CT23RO01 Rover 75 Car Stereo Double Din Fascia Panel Adaptor
@ £14.34p
The Antenna/Aerial connector needed also from ebay :~
3xISO Female to Din Male Radio Antenna Aerial Adapter Converter for Car Stereos
3 @ 99p ...
These parts are due to arrive next week. I will report back on progress after they arrive.