Digimap OpenStream WMS times out

I am having trouble QGIS and a WMS layer that calls the Ordnance Survey/Digimap OpenStream data, but however I call the layer it times out without ever displaying a map.
I have registered with Digimap and have my own API key. I have successfully created a WMS connection that allows me to call an OpenStream layer, but once I have clicked ‘Add’ to place the layer in the map, nothing appears until I receive a notice saying ‘connection timed out’. This problem occurs whatever parameters I set for the OpenStream entry (username and password, CRS options, tile size… ). I can successfully create and load other WMS connections and add a layer, such as the following: http://ows.terrestris.de/osm/service
This problem occurs in Windows and Linux versions of QGIS, and is not firewall related.