Thanks for your comments on this.
It was the wires at the rear stopping the radio coming out. I have
removed the centre console trim on both sides (one broke!) and managed
to get access to the rear wiring and released it. The radios are
obviously installed in the factory before the tunnel and footwell trim
is installed.

Still, I now have my new radio installed, a Blaupunkt Melbourne SD27
radio/SD card reader. See http://www.bluespot.co.uk/stock/melbourne.asp