SO lines are booked but header is closed - cannot progress lines manually

R12.1.3

We have a sales order that we cannot progress.  The order lines were placed on hold when the order was booked, some of the holds have now been released so that the customer can be invoiced, but we cannot progress the order to create the invoice.  We have added the scheduled dates as these were missing, but the order still won't progress.  We get an error : APP-ONT-251013: There are no eligible activities to be completed. The flow may be completed. Please see the workflow monitor or status window for more information. 

When I have looked for a workflow, there is none associated with the order - it looks to have been purged.  The order was created in December 2016, and has been on hold since.  When running diagnostics I can see that the workflow was closed on 31 Dec 2016, but at that time the order lines were still on hold.