They have upheld my complaint.
"You are correct in thinking that the invitation to Dr Williams was withdrawn after Rachel McKinnon had made that a condition of her own participation. While it’s not the case that the BBC allows contributors the right of veto, contributors do sometimes make conditions, and programme-makers then have to decide what course of action will best serve the audience. Because the starting point for the item was Martina Navratilova’s article, and because that article had referred particularly to Ms McKinnon, the programme-makers judged that her participation would add more to listeners’ understanding of the topic than Dr Williams’, but the decision was made with the intention of ensuring that listeners also had an opportunity of hearing Dr Williams’ views, preferably within the same programme. Dr Williams was then invited to be interviewed later in the programme, and accepted. She subsequently changed her mind, but by that time the discussion with Ms McKinnon and the other participant, Charlie Martin, had already been recorded. I think it’s worth bearing in mind that it was recorded in the expectation that there would be a balancing view later in the programme.
Having said that, though, I would agree with you that there should nevertheless have been an element of challenge to the views expressed by Ms McKinnon and Ms Martin (as there was in the interview with Dr Williams in the 24 February edition of the programme), and that it was missing from the item as broadcast. I’m therefore upholding your complaint on that basis".
So FiveLive never allow a contributor to veto another guest, but they do allow contributors to have conditions that other guests are uninvited. Utter bollocks.
And bloody ridiculous that you should have to go through 3 levels of complaint (and ten weeks) for something that:
a) Should never have happened in the first place
b) Should have been resolved at stage 1 of the complaints process with an apology