The problem is caused by your label tags.
<td valign="bottom" bgcolor="#F3F3F3" class="ResultsTableCell"><br />
<label for="ATCCheck">
<input name="ATCCheck[]" id="ATCCheck" type="checkbox" class="checkbox" validate="required:true" value="66" onclick="setChecks(this)" />
<span class="WADAResultsTableCellname"> Add counsellor to booking</span></label></td>
you have a few issues in your label tag:
1) the for attribute of the label tag should referance the name, in your case, where the name end in"[]", the for attribute must also end in "[]";
<label for="ATCCheck[]">
2) when using a label tag, there are two methods:
a) Wrap with label tag.
b) Attach using For attribute.
you should choose one or the other, not use both.
your checkbox code should be changed to:
<td valign="bottom" bgcolor="#F3F3F3" class="ResultsTableCell"><br /><input name="ATCCheck[]" id="ATCCheck" type="checkbox" class="checkbox" validate="required:true" value="66" onclick="setChecks(this)" />
<label for="ATCCheck[]"><span class="WADAResultsTableCellname"> Add counsellor to booking</span></label></td>