Hello,

From France too, I can tell you my experience :
I upgraded my MKIII from V20 to V24.1 a few months ago. I had the message Software update ... and nothing happens.
When I reboot the car, I had a blue screen like MSDOS, with the radio, the TV, the phone working, but no more gps.

I tried many things, but I finally bought a new MKIV on ebay for about 450 euros. Now I upgraded to V28 with no problems and I use a home made 2007-1 europe map with french radars.

I think you have to replace your mkIII.