Has no-one else found this? Listening to it now. One hour 38 minutes of Maddy Prior and guests in concert at Cecil Sharp House, recorded on 23rd October 2008.
Actually I've said it's on i-player in the thread title, but I've just realised that I found it by Googling "Electric proms", after I heard bits of it on Mike Harding on Wednesday whilst on my way to Otley Folk Club.
Just checked, can't find it on i-player, so perhaps some kind soul could change the thread title to "Maddy Prior on BBC Electic Proms".
Find it here, then click on Artists K-M.