The stock HU was only playing cd's when it felt like it.
So I popped it out and had a look on the back, surprisingly there were 3 connectors, 2 of which were standard eurodin compatible and the 3rd was presumably for the stalk control and display.etc
I went down argos (wanted it instantly otherwise id have orderd online) and bought a sony Headunit popped it in the hole plugged the connectors in (leaving the 3rd one out) and it appears to work.
Only thing I had to do was swap the Yellow and the Red wires so that it didn't lose it's settings after I turned the car off (Sony manual suggests that if you have problems and helpfully they put in-line push fit connectors on the wires on the included sony > ISO harness to make it a doddle to do.)
I then set the auto off timer to kill the display 30s after the HU has been turned off so that it doesn't stay on and flatten the car battery.
Have to manually turn it off with the "off" button at the moment but that's only a minor inconvenience.
The cars display seems to still display informational messages and the time so it appears disconnecting the stock HU didn't prevent it from functioning properly.
I do need to get a stalk adaptor if I want the steering wheel controls to work, which I might do, although that costs almost half what I paid for the HU itself.