Hi Malcolm,
The link you referenced above is the one I had come across.
However, before you commit to anything, I very strongly advise that you speak with your local Lexus dealer parts dept. to confirm the type and generation of the sat nav fitted to your car.
The information I gave is purely from memory and would need to be checked as your car may have a later system fitted. My car is from the first month of production and I know there were several updates to various systems early in the model's life.
In the end I did'nt bother with the update and simply use a Tom-Tom, with up to the minute maps, any time I need guidence. The original 2008/2009 mapping actually works quite well most of the time.
Enjoy the car.
Dec.