Hey there Simofletch.
I have an issue also with my SatNav display, but I dunno if it is the same as your model, mine is a Grandtour Sport from late 2005 with the Original Carminat dsiplay system- not the "pop-up" colour Sat Nav display.
Anyway if you look at the dash board pictures I've attached, (mine is Left Hand Drive, but I'm sure it's the same over in the UK) you will see a white line along the bottom of the display.
In front of the white line there is the seam/join where the whole housing unit joins the dashboard (it's lika an oval shaped unit). At the front edge there are 2 spring clips inside.
Push the housing up and back towards the windscreen - (gentle but constant pressure) and inside the 2 spring clips will release quite easily.
The whole unit lifts out until the limit of the cableing. Release the cabling(2 attachments with locking clips) and lift out the LCD housing.
The LCD itself is easily removed as it is only a few plastic clips that holds it in place.
Swap it out and away you go
If it isn't the same model I apologize - but maybe someone else has use for this
regardz à tous