Incorrect bill - yet again

When I took out a new contract I agreed a new 18 month contract based on the total cost of £88.50 a month.
When the agent started to process the change it was noted they had forgot about the second 360 TV box so agreed a £10 credit would be applied to my account each month to correct it to the agreed price of £88.50.

Last month I was billed £98.50, so I questioned this on Whatsapp and the agent agreed it was wrong and applied an additional £10 credit to my account.
This months bill should only be £78.50 to correct the previous month's incorrect billing, but this month's bill has arrived and it has no trace of a credit (even though I got an email confirming the credit) it again shows a direct debit of £98.50 is going to be taken.

When it's now the second month of incorrect billing I'm going to try Whatsapp again, but would like someone else from Virgin to take a look at my account as I'm not convinced it's actually going to be resolved.
As I only agreed to a new contract of £88.50 I'm really not happy at constantly being billed incorrectly.