Since May this year mediation has been mandatory. However, it is only mandatory to "attend" the call.
In these types of claims, it is a complete waste of time. However, all you have to do is offer £0 and it will be over in minutes.
It is not a part of the actual judicial process. No judge or lawyers are involved. Nothing to worry about. Just take the call, offer £0 and that's the end of the matter.
So what if it's on NYE? It's a working day. If the date and time is not convenient, just email Tham and ask for a different appointment.