Send Change to Workflow

I'm trying to send a change request to workflow using the rules in 7.5 SP1 and I'm getting a 'failed to verify SessionID' when this runs.

The workflow I'm sending it to works if you enter the change SessionID when using the debugger, so this suggests I'm missing something in Send change to workflow and the connection to the workflow itself.

Does anyone know what the Send Change to workflow sends to the workflow in terms of process information when it executes? i.e. what do I need to have in the workflow as the input for this to work? SessionID, ProcessiD?

The Send Incident to workflow uses the Incident SessionID so I presumed the Change would be the equivalent....

Thanks  in advance

Steve