I managed to get the new one in place without taking the old one out, total nightmare. I took the centre console out and spent quite a bit of buggering around trying to get the control unit out but to no avail, its a massive job to take the dashboard out from what I could see. however the plug came off with a few minutes work and the new unit sits quite happily at the back of the radio and works fine. The most time consuming bit was the ignition. I initially tried to swap all the electronics across from the new system to the old, but the key wouldn't start the vehicle so I made the assumption that the key itself must be chipped to the electronic ignition. I had to cut a slot into the rounded top of the bolts holding the ignition onto the steering wheel shaft, it took almost two hours with a hacksaw blade... but I managed and unscrewed it..
so that's it replaced and the entire electrical system working...except for the bloody mirrors which seem to randomly open and close, so answers on a postcard on that one please!!
Thank you for your help!!