Just for the benefit for anyone reading this in the future, I got an email SEVEN weeks after the Royal Mail tracking website confirmed they'd received my return package telling me I'd likely be charged for the missing equipment.
Calling them and giving them the tracking number resulted in an immediate confirmation the package had been received followed by an email confirming I wouldn't be charged. I was informed that it was Royal Mail's fault for not telling them they had delivered the package to BT. (It would appear that BT don't know what they've received until they're told it has been delivered - the logic of that defies any common sense explanation.)
Keep the tracking number and call them after THREE weeks. Something sounds broken at the returns centre and they've no incentive to fix it, I presume, whilst they think they can still bill customers for their inefficiency. Of course, this relies on their hope that customers will not make a note of their tracking number nor track and trace the package themselves.
To avoid being billed for it, if you're returning kit to them it's probably best to assume they are incompetent and prepare to evidence your compliance with their system even if they can't.
