Thank you very much! - that worked.
For the benefit of anyone else who would like to do this, I used this code:
var editor1 = window.opener.document.getElementById('CE_ctl00_maincontent_Editor1_ID');
var editdoc = editor1.GetDocument();
var count;
for(count = 0; count < editdoc.anchors.length; count++){
var oAnchor = editdoc.anchors[count];
document.write("<option value=' + oAnchor.name + '>" + oAnchor.name + "</option>");
}
}
<select name="hash_2">
<option value="top">Top of page</option>
<script type="text/javascript">
anchorOptions();
</script>
</select>