Right.
The date is now set for performance - Sat October 1st
The performance venue - London, TBA (negotiations under way)
Rehearsal and Q&As - Sat September 24th, all day.
Rehearsal venue - Cecil Sharp House
Programme - Pleasant and Delightful plus second song TBA before rehearsal.
Instruments - welcome, but must be covert and capable of being whipped out and played in an instant (thus it was with regret that we had to turn down Martin Carthy; it was the tuning, you see...).

I appreciate that the very notion of rehearsal may be anathema to many seasoned performers, but the hope is that on the day everyone can sing the same song to the same tune in the same rhythm at the same time (with harmonies for them as likes 'em).
To that end, the intention is to email out a sound file of each song together with notation and lyrics, so everyone can get to grips with it. On the rehearsal day we'll be trying to get the kick-off perfected; it's harder than you think to break into song 'spontaneously' when you're standing on your own in the middle of a crowd with precious little in the way of a cue.

I'd be hugely grateful if all those interested could PM me their email address (which will not be passed on to any third parties), and as soon as the sound files (which will be less than 3Mb) and the dots'n'lyrics are ready I'll send them over.