Hi Marc,
Thanks for that... I had a feeling it was due to the child list length.
In respect of the simpleChild: true I dont seem to have much joy with that and had attempted this earlier from your documentation.
When adding that one line to the script the result I get is that the dropdown remains with the same appearance and functionality (<20 options) but now it does not filter based on the parent list either.
Here is the script I have in place.
<script language="javascript" type="text/javascript" src=""../../jQuery%20Libraries/jquery-1.8.2.js"></script>
<script language="javascript" type="text/javascript" src=""../../jQuery%20Libraries/jquery.SPServices-0.7.2.min.js"></script>