You simply offer £0 to settle, and if the mediator wants to talk about your defence you point them to the one you have submitted and make a formal complaint after the call if they push it, you do not discuss it with them and they are not part of the legal process. It’s a box ticking exercise, and would only last an hour if you were serious about settling, but once you make it clear that you’re not then that will be that - the mediator will make your £0 offer to the claimant and it won’t be accepted. Done. Boxes ticked.