Thanks Guy's for your perseverence and patience with me - I've built up quite a collection of Garmin tools so far!
For lolypop's info (I can't send PM's yet as I'm still a 'Newbie' or should that be 'Nuvi !?!) - holding the on/off switch left didn't work. I tried holding for a few minutes - no good, same old 'blinking' white screen.

Kunix, you mentioned an update.txt file. It contains:
xrgn,14,1:/14.bin
reboot
Is that correct for what I am trying to achieve?

I unzipped the OnSDCard files to a SD card and it created a root folder 'OnSDcard'. Does the size of card matter? I used 512Mb.
Also tried with the Garmin folder and 14.bin in the root - still didn't boot up.
I see ldr.bin is a few folders deeper - should this be in the root?
I would like to try this first before I open the case and risk breaking the ribbon connectors next time!