Do you know if she was able to rebook that time to another client, Ali? If not, I think you're being somewhat unreasonable and the 50% seems fair -she was unable to use that chair/room during that time which represent loss of income to her, although she may have been able to avoid paying the therapist if she was able to get the message to her in time (which seems unlikely if i understand correctly and you called in the morning to cancel a 10am appointment).
It is standard practice here for you to be responsible for 50-100% of the charges if you cancel an appointment with les than 24hours notice, almost without exception. A child forgetting to inform you of a school obligation would not usually be considered an emergency.
Before sending this letter, I would advise you to consider what outcome you want. Do you want to get the most you can from the voucher, do you want revenge, or do you want justice? The letter is at best unpleasant in tone and makes you sound like a bullying harridan who attacks and then hides behind her husband's coattails. I believe this letter would raise the hackles of most people and cause them to give you the minimum they possibly can. If you can stand to lose the whole voucher, go ahead. It will probably make her upset/angry for a while, so you will have revenge.
If you want to see some money or other form of compensation, I advise you to adopt a much more conciliatory tone.
If you want true justice, I would seek the advice of a local business bureau/citizens advice bureau before taking any action.
this letter is written while you are still in the anger stage of your upset, and I believe that it may not be in your best interests to act during that stage.
JMHO
Dr. Monster
