Following some research and experiment I have an answer for non-technical folk like myself.
The first part of the solution seems to be to download Open Street Map (OSM) material formatted such that Garmin devices can read it. There seem to be several sources if one looks at the OSM website [https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download] or one can make use of the map cited earlier in this thread. However that requires payment to the hosting organisation to download it at anything. like a sensible speed
I made use of Open Map Chest [https://www.openmapchest.org/maps/united-kingdom-ireland/] which is run by map enthusiast Ben Konrath who offers all his maps free of charge. The UK versions are also routeable so that one can still make use of Post Code searches to find places. The site provides instructions for the downloading of the maps using a BitTorrent tool to suit your operating system (downloadable from the site if you do not have it) and a suitable unzipping tool to give you the file in its final format. Once you have followed Ben’s instruction you should end up with a file called ‘gmapsupp.img’ in a suitable folder somewhere.
The second part of the solution is to get your in-car system to accept the OSM map as an update. The specifics of this next step depend upon which version of the Kenwood head set you happen to have installed. The DNX 521 DAB does not have an SD card for mapping but instead uses a data stick which is attached to a floating socket in the glove compartment.
Step 1 – Follow the instruction in the manual for your device to get a map update. Essentially it will be a case of going to Settings, System, About, Update Map. You will need to insert a blank data stick of at least 4 Gb and the set will write a set idetifier .XML file to it. The data stick will then probably have the following file structure:
[folder]Garmin
GarminDevice.xml
It is no good adding the OSM map at this point as your set will not recognise it as an update.
Step 2 – Go to your PC and get logged into the Garmin website. You will probably be forced to load the useless Garmin Express software. Plug in your data stick and it will show you what Garmin material is associated with your device, particularly the currently installed map. You will also see that the opportunity to update your map is not available. However the option to download your existing map for re-installation will be available. Choose to do that then go off and have a cup of coffee. While you are away the map files and a few other bits and bobs will be added to your data stick.
Step 3 – When the download of the existing map is finished you should find the following file structure on the data stick:
[folder]Garmin
[folder]JVC
[folder]Pnx
.express
GarminDevice.xml
gmapsupp.gma
gmapsupp.img
gmapsupp.unl
gmaptz.img
The file ‘gmapsupp.img’ is your original map. If you want to make a copy of it now is the time. Next delete that file on the data stick and copy in its place the Open Map Chest version of ‘gmapsupp.img’. Make a back up of the data stick for future reference.
Step 4 – Take your data stick to your vehicle and follow the instructions for updating maps. This will most likely be Settings, System, About, Update Map. The update should then start and will take some time. Once finished, switch off the ignition, remove the data stick and the job should be done. When you switch on again things will look much the same as before the update but closer inspection of the map will show some slight difference in style between Garmin original maps and OSM maps plus, of course, all the latest road changes not present on your old map.
I have checked the files at a binary level and have found that the update process in the headset makes no changes to any of the files on the data stick. Thus the structure of files from the data stick that you have saved ought be useful the next time you want to update. However, I have yet to test that theory.
Enjoy!
Pete