My Sat Nav suffered exactly the same symptoms after a car wash recently.
I removed the Navigation Computer drive, and found the bottom of the bracket was wet (as was the CD unit itself)
When I removed the bottom casing, I found this:
The reason the display looks different, is that the nav computer normally supplies all the screen menu's itself, but when the Sat Nav unit goes off line (or dies), there is a back-up menu function contained within the video module in the spare wheel well, that takes over with a cruder display.
Time for a replacement Nav Computer I think Andy, and yes a BMW unit is a direct swap but as you have probably got a Mark II computer, you'll need to disconnect the GPS Receiver unit (or Trimble) before you can use the BMW one. It's quite simple to do, don't worry. If however your dead nav computer was a Rover Mark III, it's a staright swap over.
the Mark II looks like this:
and the Mark III looks like this:
I can explain what to do, if you get to that stage later on.