I think I understand - it is installed, but not the version Mappero thinks it should be? So, I've uninstalled Mappero, and chalked that down to another piece of mapping software that is no use on the n900. Just as well I found a cheap n810 with car-mount kit to mess about with - looks like sat-nav might work on that? If not, I guess I'll need a flash-disk for the TomTom when we move, I guess.