Yes, another weekend with two events near each other that sound very attractive on paper. As far as guests go, anyway, one's spoilt for choice, both having brilliant lineups.
But for ye humble floorsinger, well it's all a bit different it seems ...

None of the Carsington publicity mentions floorsingers or singing sessions as such. But this year's flyer does list two "Courtyard Session" slots - these are described as "Hoi Polloi and Friends" on the Sat 1300-1600 and "singers and musicians welcome" on the Sun 1230 onwards. From what I remember of a previous year however, this was mainly a musicians' playalong sess, so I don't know whether it's different this year. (And at risk of seeming ignorant, who's Mary De Ville?)

Southwell is even more strange. The website coyly lists "singarounds, sessions", but telephone enquiries made to the number given on the website, by other folks I know, could only elicit the mention of one singaround-type event all weekend, that being at Sat teatime for just a couple of hours, somewhere in a pub, possibly the Bramley (? - dunno, Directory Enquiries hadn't got it listed), run by festival guests (ie in between their concerts), and a musicians' session (Irish) in "a.n.other pub". That's all folks, as I understand it. (Again I may seem ignorant, but who's Mike Green?) The bits Villan refers to are down as Open Mic in Marquee 2 on the website programme I believe.

This seems to be a recurring problem, finding out about anything other than the main, ticketed concerts/events at festivals in a town you don't know.
It's simply not worth planning to make a long trip up/down/across to an unknown festival, just on spec and/or to end up wandering round all day/weekend trying to find opportunities to sing. Invariably one ends up at the wrong festival/wrong place/wrong venue/wrong time and is greeted with "oh how fantastic XXxx was" the next day on Mudcat - somewhere completely different and/or miles away but you could and would have gone there instead if only you'd known .. (ok I know a lot of good stuff happens adhoc at most festivals too.)