Thanks @garmin_nuvi for the info.
I do have this ISO (downloaded a decade ago when I got my first Garmin GPS) but have always used IMG files and wasn't aware it was possible to create them from the ISOs. I'm sure I will work it out, now I know it is possible.
I have downloaded some IMG files based on opentopo data (which are at least up to date) but the detail seemed a bit lacking. I think I have been spoilt having OS maps for the UK. I've never used maps for another country and my expectations may be too high. I was interested to see if the Garmin maps are any better but I suspect they won't be. I know it is very old but as I am using it for hiking, that isn't so important.
Device is a GPSMAP 66sr so space isn't an issue.