Ok this seems to be down to how I format my XML spec file.

-- Current -----------------------------------------------
<main>
<output>
  <name>StdQT</name>
  <desc>Standard Quicktime</desc>
  <spec>320x240 using square pixels, no crop and 3D LUT.</spec>
  <frames name="FullAp" format="2048x1556">
    <script>...</script>
  </frames>
  <frames name="Vista" format="3072x2048">
    <script>...</script>
  </frames>
</output>
</main>

This does not work, but if I format it like so;

-- New ---------------------------------------------------
<main>
<output name="StdQT">
  <desc>Standard Quicktime</desc>
  <spec>320x240 using square pixels, no crop and 3D LUT.</spec>
  <frames name="FullAp" format="2048x1556">
    <script>...</script>
  </frames>
  <frames name="Vista" format="3072x2048">
    <script>...</script>
  </frames>
</output>
</main>

This does work.

Thanks
wmw