Just had a reply from the HERE support team,
"We are aware of a technical issue that’s preventing us from offering the HERE apps in the Windows Store for new Windows and Windows Phone 8, 8.1 and 10 users. We are working to resolve the problem and restore the availability of the HERE apps for new users. Until it’s resolved, we recommend that new users take advantage of the pre-loaded Windows Maps application on their Windows operated device. If you already have the HERE apps on your Windows 8 or 8.1 device, then you should continue to receive updates and see the HERE apps available in the Windows store. Until we have a resolution, we recommend that you do not delete the apps from your device."
Hopefully it will be fixed soon. It is the best map app for travelling with - either on a phone or pad.