@homeassistant tried rebooting my Home Assistant Server again and the Hive Entities are showing as available and appear to be working again. So near enough certain the issue was at Hive's end but neither the app or their (frankly useless) customer service team could give any indication what the cause was.
IS there any #OpenSource smart thermostats and boiler interfaces available that are compatible with #Homeassistant ? if not open source, are there any that at least aren't cloud connected?