"I cancelled the appointment then received letter stating we hadn't attended"

I phoned to cancel my son's fracture clinic appointment and after repeatedly not getting an answer left a message on their machine as it instructed. I telephoned again to ensure that they had the message and left another message cancelling the appointment 

On return from holiday I received a letter stating that we did not attend the appointment. Obviously I was upset at this as I did not want an appointment to be wasted and wrote back advising as above to which I did not receive a reply. I wrote again a couple of weeks ago to which I have still not received a reply.

I am upset because I did what was asked of me and upset that this appointment could have been used for someone else. I stated in my letter that their system could be faulty and asking that they check it to ensure it doesn't happen again.