Dead easy,
Using normal car radio extraction keys, (u-shaped pieces of thick wire) take out the sat nav unit situated in the glove box, fits into the little holes on either side of the unit. BEWARE... ignition must be turned off for some 15 minutes before doing this.
Take off the connections from the back of the unit. (tricky on one of them but can be done; little catch on top middle folds up).
Leave connections off for approx 1 minute and re-connect. Push unit back in and switch on ignition. Turn on sat unit and wait. Hey presto it will come back on, fully working. go for a drive and within fifteen minutes (the time it takes to pick up sat info' as it says in the book) The sat nav will be putting you back on the right road again. This works the same as a computer re-boot does sometimes when your computer "locks up". It works!
Brian Shuttlewood, March 2009