Some fun debugging later. It turns out that guvnor is shipped with a default stencil set:
stencilsets/bpmn2.0jbpm/stencildata/bpmn2.0jbpm.orig
It is this that other posts have been talking about saying it contains German characters.
It also contains an English character that, as it turns out, the designer code that creates the bpmn2.0jbpm file specific to someones BPMN2 file cannot cope with.
It appears the wonky code that can't parse funky characters is:
com.intalio.web.preprocessing.impl.JbpmPreprocessingUnit
I haven't worked out what's wrong here.
However I will upload my nice and clean of funky characters version of bpmn2.0jbpm.orig.