a copy of the invoice (to prove purchase)and presumably you can get that from whoever bought the printer - Parcel Force won't process the claim otherwise.
In addition you should provide a print from a website that clearly shows the current replacment cost.
What I can't understand is this business of ParcelForce having no record of the consignment. All parcels are barcoded by them, so they can be tracked as they enter (and hopefully leave) the ParcelForce hubs, and it's most unusual for a package to have escaped that process. You should have been provided with a tracking number when you consigned the package, so you could track it through the ParcelForce web site - did you not get that?