Thanks guys, I appreciate all your quick and informative responses

. I checked the Hotkey 'Shift-Enter' settings in preferences as per Sig.Ovia's suggestion - Good idea - although the hotkey was still set correctly.
However, in a weird turn of events, the 'Edit' window is now working. But, there is something different about the way i'm viewing MB this time so i'll describe what's changed and maybe you guys might be able to figure out what fixed it.
I'm currently working remotely, so instead of sitting in front of my primary office computer with two screens, i'm remotely controlling it via a Windows RDP session. Naturally this locks my session down to only one screen, so could this dual screen thing really be the cause perhaps? If so, it's strange that it didn't work when I disconnected my second screen earlier.
So far it works fine as I use MB via remote RDP session. I'll be keen to see how it works when I return to my office and use MB sitting in front of the computer with dual screens.
If I continue to have the issue, i'll try Steven's fix. :-)
I'll keep you all posted once I've tried it.