the goods are returned at your risk, and the risk remains yours until the package is received by the supplier.
If somebody at the supplier's premises signed for the package it doesn't matter whether that person was authorised to do so - Parcelforce obtained a signature, and the goods have been delivered. It's not up to you to provide any further proof of delivery.
Write to the company and tell them that you expect them to meet their obligation to you in terms of a replacement or a refund. Tell them that you have evidence of delivery, and that unless you receive a confirmation of replacement/refund within seven days you reserve your right to take further action for recovery.