Touch screen has been unresponsive in the past but always functional when warmed up, but today it looks like this surely this isn't just a cold/damp issue.  If it is toast can I replace the head unit from another car without any issues. 

Cheers .

Yes - its toast.  The screen driver has probably failed.  You can either get a replacement screen - the headunit in the glove box will be fine.  If you need to get a replacement system, why not upgrade to either a SOP4.x MIB2.5 Bolero or Amunsden unit.  It will need to be coded in though for your car.  Speak to @pab567 who can help.

You can replace the screen with same part number with no coding required! But as varaderogguy said if you want to upgrade to newer models you need experts's help!

Or take the plunge and change to an Android head unit like I have just done