Just received the email for December PM and it said I used 13.69% over the month. This does not match my spreadsheet that calculated 12.70% for December.
By backwards analysis, it appears that they are including Boxing Day as a normal week day, as if I force my spreadsheet to include it, it totals 13.70%, 0.01% off their email value.
I called ovo and they have opened a ticket to investigate and a complaint as something is going wrong with their system.
Their call handler worryingly stated that bank holidays are not excluded, so I swiftly pointed them to their own T&Cs, which they then agreed with, but then stated Christmas day is a national holiday and not a bank holiday, to which I directed them to Gov.uk which says it is a bank holiday too.
Please check your spreadsheets to make sure you haven't been wronged too!