Non-billable Field Service Call showing as Invoiced and unable to edit the job

We have a couple of service jobs that were closed and the needed some changes made to them.
The ‘Billable’ wasn’t ticked so no invoice was generated however when we try to open the job and the service all in order to edit it we cannot make any changes - we get the error message that ‘The Service Call has been invoiced. Entry not allowed.’

Any idea how we ‘un-invoice’ this?