www.intelligentanswers.co.uk’s server IP address could not be found.
I disconnected from wifi and tried using the 4G EE connection - and sure enough, the site is available.
I tried using a VPN from my computer to pretend I was in San Francisco instead of York. I could see my site.
So, the web host's suggestion was that the IP is was being blocked by BT. Sure enough, I checked with the friend who initially highlighted the problem, and he confirmed that he is also using a BT connection to get online.
How do I get BT to unblock my website's IP address? It is 18.104.22.168