If the item has arrived at it’s intended destination (ie the buyer is lying) then a theft has occurred. OK, so technically it’s actually ‘Fraud by false representation’ but you get what’s meant. Parcelforce should be able to say whether the frame was ever delivered, if it was and the buyer is saying otherwise, somebody is telling porky pies and the frame has been nicked by someone.
If the buyer is indeed being honest, then Parcelforce have lost the frame and they are liable, I’ve got a horrible feeling the standard uninsured value they put on things is something like “Not exceeding £50” though. 🙁 I’d still be tempted to involve the Police even in this case, however. As totalshell pointed out above, a mountain bike frame doesn’t just get misplaced under a stack of envelopes or at the bottom of a mailbag. If it’s gone missing, then somebody in the postal chain has half-inched it.