I have successfully created a QGIS project, added a OpenCyclemap layer and uploaded the data to QGIS Cloud, and published the map. When viewing the map in a browser, the OpenCyclemap background has diagonal text across it saying "API Key required" See http://qgiscloud.com/smoky/Derby_tube_map_cloud for example.
This is the same situation on more than one map. I have another map using OpenStreetmap (created a lot earlier) which doesn't have this "API Key Required" text. This might be due to the different background or the age of the map.
Previously, I don't think the OpenCyclemap based maps had the text (although can't absolutely check).
What API Key is needed?
Where can I obtain the key?
How do I "install" the key into QGIS or QGIS Cloud (or elsewhere)?