Update :- Managed to invoke the workflow (needed to port the service selected in the parent change into the new change).
However I've hit a new snag, that when the parent change was created during the progressive capture stage the requester selects an individual that they want a certain authorisation task to be assigned to, however when the duplicated change gets to that step in the workflow it understandable falls down. Is there a way of taking the PC questions with the duplicated change? I've tried mapping the user selected in the PC to a custom field, copying that into the new change and then using that to drive who the task is assigned to, but it doesn't work, I guess because it's the handle you select at the PC stage but HB needs to know the ID when assigning a task?