I might be wrong, but I'm sure I remember my billing overview -
https://my.virginmedia.com/billing/overview - showed me the £50 credit they applied to my bill (as a correction) a few minutes after I finished the phone call with them... That's as far as my knowledge goes. It does also show unbilled usage since your last bill, though I'm not sure what else that includes aside phone calls.
I'll be chasing them for another £40-60 credit tomorrow, and will see how that shows up exactly.