Gareth, I think it would be best to get complete class(es) that are responsible for managing sessions as I can't get full picture of what you have there.
Looking at the code of startWorkflow first of all I don't see when do you create or load session. Another thing is that you use CMTDisposeCommand and you execute it on ksession - where does the ksession come from, could it be that it refers to another session? Might be better to execute it on ksession that you can get from ProcessCompletedEvent as KnowledgeRuntime.
HTH