I have tried printing a Parcelforce shipping label and am seeing an error returned:
Shipment number XXXXXXXXX not found
The error means the original label can not be reprinted after 2AM of the same day when it was generated. If you haven't shipped the order, then you would need to cancel the original label and generate a new one. This is due to Parcelforce archiving the labels at around 1.50 AM each day, which makes it impossible to reprint them.
*Parcelforce only charges for the consignments that have been scanned, so there should be no issues with double-charge.