What's your billing period? It's shown on the bill, I suspect it's somewhere around the second week of the month for you and that 76p would cover the week or so from the date you requested the increase to the date your bill is due.
My currently displayed bill is the one that covers the Oct/Nov, so I wouldn't pay the planned increase until the next bill that covers the Nov/Dec period and was due in the next week or so.
Also the maths showed it was actually based on the incorrect change to my monthly payment and covered the period between my change and the time my next bill was due, plus the guy I spoke to also confirmed it was related to the incorrect increase based on the new speed, not on the planned increase.
There is an easy way to check whether they've increased your bill incorrectly, login and click on My Upgrades and Offers in the top menu bar. This was showing that I was now on the Vivid 200 package with the full higher price.
Checking that same page now it shows the correct amount I should be paying.
Edited by deleted (Wed 18-Nov-15 20:35:31)