Master Haworth, in:
(1) Savings Advice Limited (2) Zinc Consumer Limited –v- EDF Energy Customers PLC  EWHC B1 (Costs)
found that factual statements as to the amount of costs used in mediations could be used in subsequent assessment proceedings.
I can see why that might be just in the circumstances of that case, but once someone can decide what is and what is not privileged, lay parties may well clam up, or choose to use it as an excuse to go to trial.
It's clear from
Farm Assist Ltd v DEFRA (No.1)  EWHC 3079 (TCC)
that not only do the parties to a mediation have privilege between themselves, but the mediator also has privilege, and therefore it could be argued that even if the parties litigate or agree to a disclosure the mediator would have to agree or be joined in.
Master Haworth also highlighted:
“at a mediation each party should be aware of each other’s costs at the beginning.”
which task is often achieved by the mediator.
I, and many other experienced mediators, use a demonstration known as “circles and squares” that sets out costs and risk at the beginning of the mediation. Whilst it's not without controversy, it focuses the parties’ minds, and I as mediator destroy the demonstration at the end of the mediation it may be a good way forward.