An update seems to have broken the servers.
The following is the conversation I had with Whitey on steam this morning:-
QuoteWhitey: although Boomer on Standard SE collection, you can change to any map on the server including the ones in our collection. Wish Valve would test updates before releasing them :SBlunt: So does that mean it's working normally now? and thanks for sorting it out
Whitey: No
Whitey: Workshop maps are not working correctly
Blunt: ah
Whitey: well... the collection is
Whitey: Boomer works fine on the SE collection but if you want to use anything other than standard SE maps you need to changemap manually
Whitey: I've left MH on our collection for testing but if any of the admins restart it from HLSW it won't come back
Blunt: ok
Whitey: It needs someone to change the map on the server console to kick it into life
Whitey: and if what i've read on forum is right then it could go away again at map change
Whitey: so best play on Boomer until Valve issue a fix