Thanks for the suggestions.

I have oredered another microSD card to see if that helps at all.

In the meanwhile I reformatted the existing card and re-installed the map using Garmin Express (the map had previously be re-installed using MapInstall) but still couldn't be read on the Nuvi.

I don't know what the signal path is on the Nuvi but when the map can be read on Garmin BaseCamp via the Nuvi it must be lifting the data off the card to send via the USB computer connection but for some reason it's own operating system isn't able to access the same data.

When the new card arrives I'll format using the SD Association tool (thanks for the link) then report back.

Thanks