Okay, so I thought I'd try and collate all the tweets over the last two weeks (the time/date stamps might be a little off as I'm in Australia and the tweets were made in the UK, so the GMT stuff might have shifted things about a little, but you should be able to get the general gist):
Oct 9: Katy Manning said she had "done one of [her] favourite @bigfinish audios last week", which I don't think we know anything else about?
Oct 9: Katy Manning also said she was "readying this weeks with some lovely surprise people".
Oct 12: John Dorney teased a script with "an astonishing cast", which David Richardson strongly suggested was related to
Ravenous 3, later calling it a "special day" (Oct 13).
Oct 13: Rosalyn Landor was recording a 4-part story (2 days of recording) with Katy Manning and Jon Culshaw in a story likely written by Guy Adams and directed by Nick Briggs.
Oct 13: John Dorney confirmed recording to take place "on Monday" (Oct 15), and likely to be announced sooner than the probable
Ravenous 3 stuff*.
Oct 15 (tweet not in this thread): Katy Manning confirmed she had been recording that day and would be back in the next day.
Oct 17: Jon Culshaw said he had been playing a "legendary character", saying "Well, here we go again...", strongly suggesting that his recording with Katy Manning is as him playing the Brigadier. This would mean that the Brig is appearing in both stories of a probable
Volume 5 of the
Third Doctor Adventures, one story written by Guy Adams and the other by John Dorney, as confirmed by Dorney saying it was a "delight to work with [Jon]" on the same day.
Oct 17: Bethan Dixon Bate said that she was also involved in the Mon/Tues recording.
Oct 17: Angus Dunican teased he had written a story to be recorded the next day (due to GMT differences, this recording was likely Oct 17-18).
Oct 18: Sarah Douglas confirmed she had been recording a 6th Doctor story with Colin Baker, with Nicola Bryant later confirming that she too was involved in that 2-day recording.
Oct 18: Ryan Forde Iosco said he had just come back from recording, strongly suggesting he will appear in the 6/Peri story.
EDIT: Oct 18: Scott Handcock appeared to have been involved with Big Finish goings on for a couple of days, so he was likely involved in the 6th Doctor/Peri story recording (possibly as the director?).
*My personal theory is an announcement for the
Third Doctor Adventures: Volume 5 on November the 9th, the 50th anniversary of the appearance of BRIGADIER Lethbridge-Stewart.
What We 'Know':-Katy Manning has recorded something we don't know anything about yet.
- There is a very good likelihood Jon Culshaw has been cast as the Brigadier in a box set of
Third Doctor Adventures, due for announcement sooner than the
Ravenous 3 details. The stories are written by Guy Adams and John Dorney and are directed by Nick Briggs. The first story will guest star Rosalyn Landor, and the second story will guest star Bethan Dixon Bate.
- There is a 6th Doctor and Peri story coming out written by Angus Dunican, possibly directed by Scott Handcock (?). It will guest star Sarah Douglas and Ryan Forde Iosco.
I think that just about covers everything