Molly
You can move the appointment once but then the claimant must attend the new one or risk their claim being closed. I would call the day before the current booking (if possible), to cancel and then make a new appointment while on the phone for a mutually agreeable date, if they are not able to offer a date on the phone then make sure that you give then any dates that are not suitable and then confirm these in writing.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
