The letter from DCB Legal is normal and expected. Nothing to do but file it.
For claims issued after 22nd May 2024, mediation is not optional any more. It is no big deal and is not part of the legal process. It is not a trial and no judge or lawyers are involved.
The only requirement is to "attend" the phone call. You do not have to agree to anything in the mediation and you should offer £0. It will be over in less than 5 minutes. You will have "attended" the call and that is all that was required.
The mediator is not a person with legal training. They will have had a days course on how to mediate over the phone and that's about.
You are waiting for your N180 DQ. Download one from here:
https://assets.publishing.service.gov.uk/media/66a8b019ce1fd0da7b592f43/N180_0724.pdfFill it out and sign it by typing your name for the signature and keep it handy for. when yours arrives by post. You won't use the paper one. The one you download and complete will be emailed as a PDF attachment to dq.cnbc@justice.goc.uk AND to info@dcblegal.co.uk AND CC in yourself.
Don't send it until your MCOL history says that one has been sent to you. No need to wait for the postal one, just send the one you have already filled in as above.