sn07

Thanks very much. It helped put me in the right direction. I had to make a few changes as it did not find the function at first. However I found that I need to use the id everywhere and not the name. So the final code is

form-topic.php

<div class="bbp-submit-wrapper">
<form id="frm" name="frm">
<input type="checkbox" id="chk" name="chk" onClick="apply_submit()">Tick to accept
<button type="submit" disabled="disabled" tabindex="<?php bbp_tab_index(); ?>" id="bbp_topic_submit" name="bbp_topic_submit" class="button submit"><?php _e( 'Submit', 'bbpress' ); ?></button>
</form>

functions.php

<script type="text/javascript">
function apply_submit()
{
 if(document.getElementById("chk").checked==true)
  {
    document.getElementById("bbp_topic_submit").removeAttribute("disabled");
  }
 if(document.getElementById("chk").checked==false)
  {
   document.getElementById("bbp_topic_submit").setAttribute("disabled", "disabled");
  }
}
 </script>