@ pe1agp,
thanks for your advice. Please excuse the tardy response, I think we may be in different time zones!
Sorry to say I don't understand your statement "put the correct meta.txt in the folder in which you patch the maps". I guess this is not the TTActivator folder, but I cannot see how to get TTActivator to look in a specified location for a file. If I update the meta.txt file in the TTActivator folder, it is overwritten by the one from the TTActivator server when TTActivator is opened.
I have also downloaded FastActivate from your post but it doesn't open in my windows 10 PC, just flashes up a DOS box and then closes. In any case it seems from the instructions you need to be able to see the device as a drive in windows explorer, and these NAV3 devices don't show up there.
edit: All sorted thanks for all your help. The clue was to understand that what the error message was telling me was that the map hadn't been activated. I installed a fresh copy of TTActivator onto an old laptop which I then quarantined by disconnecting it from the net, then used that to activate the maps on the 2 devices. No idea what the actual problem was, possibly something going on in my desktop PC.