Well, I've had a bit of a fiddle with it today and had no luck what-so-ever.
To get to the unit, you have to take the front seat out and then remove a black box that the unit sits in. Stripped the lid off the sat-nav unit and gave the lens a good clean, tested it while everything was stripped out and it hasn't made a blind bit of diference.
The disc goes in, the head attempts to read the disc a couple of times then the disc gets spat out. Message on the display "No CD inserted"
I've just put everything back and now the airbag light is permanantly on. Great eh!!!!
Does anyone know if the light just stays on after a lead has been disconnected and it needs to be reset? The plugs are back in correctly and no force was needed to remove them, so I doubt the wires are damaged.
*Edit*
Just read the Airbag warning light thread and my question has been answered. The computer has detected a fault (i.e. Me messing around) and the light will be on until the dealer can reset it. I'll wait until I've actually fixed the sat-nav unit before I pester the dealer.  | 
