Thanks, it was the fact that I had to enable it twice that threw me.
It seems that there are two types of POIs:
Enables the POI's icon to show on the live maps. If not enabled, you can still navigate to this POI but you will not see it when driving past it.
2..Preferences\Navigation\Custom POI Alerts.
This option enables your POI to have an Audio Alert and a Visual Alert as you approach it while navigating.
It seems that all custom POIs appear in #1. If they are not enabled in #1 then they will not show on the live maps (as icons), even if enabled in #2. However, you will still get Visual & Audio alerts as you approach these. You can also still navigate to these POIs by selecting Places Of Interest\Nearest POI and selecting the desired POI.
Seems like a very awkward way to manage POIs IYAM (If You Ask Me)! I think the Navman designers could have worded the menus better. Like the menu for #1 above should be named "Enable POI Icons on maps" and for #2 "Enable POI Alerts".
Note: To use the Postcode POI, select Places Of Interest\Nearest POI and choose Postcode then click the tick mark. Now enter a postcode and it gives you the suburb. You can now select this POI to navigate to!