Capture Message ID for (IDOC-XI-HTTP)

Hi! Thanks to some kind help last week, I can capture the message ID in my mapping via the following:

*******************code**********************

java.util.Map = container.getTransformationParameters();

String mid = (String) map.get ( StreamTransformationConstants.MESSAGE_ID);

return mid;

*********************end code******************

However, my scenario is IDOC->XI Multimap->File(s). In the case that the IDOCs are mapped into multiple files, each file has a unique reference associated with it. I can see these in the HopList as follows:

<Trace level="3" type="T">clone_MMF 453D478CFCF537B0E10000009B7D1E4D</Trace>

Does anyone know how I can capture this reference in the mapping?

Help is much appreciated and rewarded. Thx, Duncan