Thanks for the above advice; I have the HeliRoutes working/displaying on the unit now.
The solution was to send the files via Garmin BaseMap to the unit (not unit card, as it give two options). Then you go into Tools/Tracklog and then tick “show on map” for each route you transferred to the unit. After restarting the unit, the HeliRoutes (or whatever routes you transferred) will now show on your map.