Being charged for returned equipment

[MK28]

I returned all equipment when my contract was up back in October. I had confirmation that the returns box was received but they are saying that the broadband booster was missing and now want to charge £33. 
I know that everything I had from sky was in that box and believe that it has been an error on skys part when collating the equipment returned. I have no use for a booster that isn't compatible with my new broadband. But I have no proof as to what was inside the box that sky has received so at this point it's my word against theirs. I have spoken to customer services through messages previously but with no resolution. 
Any help gratefully received! 

[caesarome]

@MK28 

I take it you didn't take a photo of everything that was inside the packaging before you returned it, if not you could ask them for proof that it wasn't there to set if they relent.

[MK28]

Thanks for replying. No I didn't take a photo, I just slotted everything in the holes and sent it. I wish I had! 
I can't imagine sky will be up for sending me a photo of what they had received either. 
They are now threatening to send on the debt to a collection agency if I don't pay within 10 days. 
Very frustrating considering the thousands of pounds I paid to them over 16 years of being a 'valued' customer.