Netflix charging issues

My virgin package apparently includes Netflix with ads and I can upgrade to standard without ads or to premium.  I thought that I would be charged a top up i.e the standard without ads less the usual cost of the with ads version.  However I have been charged the full cost of standard without ads by VM.  I have queried this with VM and told it is correct that I should pay the full price and not a top up fee charge.  

It doesn’t seem right to me, so I did some reading in this community and found that another VM customer paid only a top up fee.  I was advised by VM to take it up with Netflix but surely it’s a VM billing issue?

Anyone got any experience or advice on this please?

Many thanks

Joanne