[quote=justpootling;36406092]I've always assumed the musical guest performances are pre-recorded and then edited in to the live show.
Justpootling - I was there and they were filmed sometime after 6pm on Sat night (evidently the sound had been recorded earlier that afternoon) before the main show went out live. With some exceptions (eg Spandau), this is normally how it works. All the equipment etc is then cleared away ready for the show to start and as you rightly assumed, the performances are then edited in to the live show later on and the audience watches it on the monitors.