Hi Hamish,
I'm picking up your posting on NZEUG and Geonet regarding not being unable to add a geocoder to ArcGIS Online. I tried the same URL here and it worked almost instantly. The URL itself looks OK, I can access the REST Endpoint and it appears to be fine.
Are you adding the URL as a general item to My Content, or as an organization geocoder in My Organization > Edit Settings > Utility Services > Geocoding?
Can you try adding it again and see what happens, in case it was a temporary problem?
Can you log a support call with Eagle and we can take this further?
Regards,
Malachy Carey
GIS Technical Support
Eagle Technology Ltd