Incorrect billing

Can someone from Virgin help me please.  After problems with my TiVo box an engineer replaced it with a 360 box.  He took the defective Tivo box away with him. A short time later I received an email saying that I had to return my Humax box or pay a fee of £90.  Initially I was confused as to what a Humax box had to do with Virgin and it was only whilst I was on the phone trying to sort it out that I happened to look at the label on the underside of the 360 box that I found that in fact it was manufactured  by Humax.  I explained to the assistant that the Hunax box was the new box supplied by Virgin (she didn't seem to know) and she said that she would sort it out and I should disregard the email. A month later I got a similar email, I again contacted Virgin and was told it would all be sorted.  A month or so later my 360 box developed a fault (it would not let me record any programmes).  I was unable to resolve the issue and so I called an engineer out.  He resolved the problem by resetting the 360 box and it now works fine.  I mentioned the emails to him, he looked at my records on his tablet and he said something had been mis- recorded  but he would sort it out.  Well guess what I have now had £90 added to my latest bill because presumably I still have not returned my 360 (Humax) box.  How many times do I have to tell Virgin about this problem?  Can someone advise who to speak to make sure that Virgin's  records are correct and I am not repeatedly ask to pay for equipment which I am already paying a monthly fee to rent.