I opened an existing score tonight and none of the notes in it have Sibelius' Live Velocity and Live Start Position options available; they are greyed our and all I can select on the ";Playback"; tab is ";Play on Pass"; and ";Fermata";. If I open a clean version of the 6.0a template and input a few notes, these notes have the options available as I would expect.

Does anyone have any ideas as to why this might happen and how I might fix it? I need to get some cymbal rolls in the right place and the problem is hampering my workflow.

Make sure Play > Live Playback is checked. This is document specific, not global, so every score will save those settings. You must have switched that off by accident.

Thanks; that was the problem. I wasn't aware of that option.