I'm in the same position, awaiting a bill since August, and it has happened a few times before. Drop them an email and they will resolve it eventually in my experience. On previous occasions, they have been missing some 30minute billing data from a specific day, and it seems they need to manually re-request it from the central billing database that is outside of octopus control, so I think the problem is down to issues with Octopus receiving the 30minute data in the first place.
Mine was waiting on missing data from 20 September, this was resolved but then all data flow stopped on my account for a couple of weeks. This is all now showing apart from one day in November, so am waiting on them to catch that up manually too. They will get there I'm sure, and frankly given I'm on the 13.8p/5p rates until end of April I'm really not going to complain if it takes them a while, there's so much benefit from having Go that I'm more than willing to live with the beta aspects of the service.
I do regular exports of usage data from Octopus Watch app to a CSV file as an emergency backup, but have never needed to resort to this.
