Function Keys no longer available for Switching Workspaces

For about 15 years I have assigned a workspace to each function key (F1-F10) in whichever Linux/X environment I'm using. Yesterday I ran a normal software update on my work Ubuntu box and now, although I can still use ccsm to assign those keys to the Viewport Switcher it has no effect unless I also assign some modifier such as alt-, which is very annoying. (I'm using metacity on 14.04). Does anyone know what might have changed in the last couple of weeks? I had been on holiday so the update was quite large. More importantly, how do I fix it?