Hi Luke20,
The workflows need something to trigger on. Usually, that's a save or create. If you don't have that step, the workflows may not run. Of course, you can manually run the workflow on the forms, but that may be too onerous. To make it more of a batch, you could create a separate column or list and edit it in a data view and that could trigger a workflow on old items. The batch edit step would still be manual but it would be a lot less work than starting a workflow for each of the old items. On the other hand, if you only have a few old items, just have the admin manually run an "Overdue" workflow on those.
We have a product called DBXL that has support for sending out notifications when a form exceeds a due date. If third party software is an option let us know and we can help you configure that.
Sorry I didn't have a silver bullet,