I got it.

I registered the GPS which I hadn't done the first time. The information on
finding the serial number in that site section is more complete but still
wrong. It says it is on the back but it is inside and is the sticker I
first used. I also appeares on a very small sticker on the top (not the
bottom) of the box that doesn't look like the sticker they show a picture of
on the web site.

Anyway, I went through the routine of unlocking for a second GPS (you are
allowed two) and it worked. I have my GPS working but now I have to call
Garmin and find out whether they think I have two units and see if I can
pursuade them to delete the first, ghost, unit so I can load the charts on
the backup GPS I have yet to buy.

--
Roger Long