Really hoping this is an error, not my hosting company getting hacked, as happened a couple of years ago - although they fixed it in a couple of hours then.
The links that do this are
I already know that I'm not getting this message on an HTML file somewhat below the top level, which is not called index.htm, so it's possible that someone has hacked the hosting company and replaced all the index.htm files, as happened last time.
Later - have a little more confirmation that there's a problem. Please DO NOT try to access the site!
Later still - as suspected, it was a wholesale replacement of index.htm across the site. I've replaced them all as a temporary measure, there will probably be a few bugs so please bear with me while I sort things out. Hopefully the immediate problem is sorted.