Jan
There are no circumstances where you can be requested to attend a new ESA medical, without an ESA50 having been issued, although it is possible to have one booked if the ESA50 has not been returned.
I would suggest you contact ATOS and the DWP, to let them know that you have not received one, and ask for the medical to be deferred until you have had a chance to complete and return an ESA50.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems