Hi Robert,
As you may have seen we’re getting a lot of these queries at the moment.
From what you’ve said I think it’s likely that you’ve been charged the zone 1-6 daily cap plus the Maidenhead to West Drayton peak single and an off-peak extension fare from zone 6 to Maidenhead. The problem is that no-one publishes the extension fares so it’s difficult to tell. If it’s as I’ve suggested then the extension fare is £2.20 which seems a little low. It is possible that the first fare has part contributed to the cap, but I’m not convinced that is necessarily the case. If a smaller cap (eg zone 1-5) is used then there would also need to be an extension fare on the West Drayton to Canary Wharf journey.
At this stage I can’t offer much more. I am trying to get to the bottom of these extension fares, so watch this site in the new year.