I have a partial solution here, that might be useful to know. Pretty much all Smart Meters store the reading history locally on the meter itself for up to 13 months - with the oldest data being overwritten once the storage gets full. This applies regardless of whether it's a SMETS1 or SMETS2 meter and can be retrieved by the supplier at anytime using SMETS Commands. You could think of it like a backup copy of everything that should have been sent to the supplier. You might also be able to retrieve a copy of it for yourself onto your smartphone or tablet.
There's an app I've heard of called Hildebrand Bright, which is developed by Hildebrand (a well known IHD manufacturer) that can access Smart Meter data for S2 meters and S1 meters that are now connected via DCC (to reassure moderators here, Hildebrand Bright is registered as a SECAS Approved DCC Other User), but it can take several hours to several days to pull all the data if the meters have been active for a while. I've never used the app myself and can't vouch for it, but you're welcome to give it a shot if you'd like to.
If you do decide to use it, the data that the app will show will be raw usage data retrieved directly from your meters and NOT from E.On's records. As such, it's likely to be slightly different since it probably won't include any corrections or adjustments that were applied later on, this is to be expected. On the plus side, the export process will still work even if E.On's systems are throwing a hissy fit, because the app pulls directly from DCC at the same level as suppliers would anyway.
I'm great at bug hunting and have seriously impressed my actual supplier with my skills over the last year. You could say that I've... Been a real Bug Blaster... However, since my actual supplier is completely unrelated to E.On, I can't throw my skills at the Kraken instance that's being used by E.On Next. Shame really. I could really contribute a lot towards breaking and fixing it if I wanted to, but I'm enjoying bug blasting elsewhere too much to want to switch tracks.
Anyway, before I get too side-tracked, I just wanted to sign off by mentioning that regardless of what the supplier does with your meter/usage/billing history in your online account, they can never modify the locally stored copies on your meters themselves because they're basically read-only once saved. And besides, there's no SMETS Command that allows the data to be modified anyway - and no SMETS Command for it means that it's basically... Impossible for the data on the meters to be tampered with remotely (proving a perfect Single Source of Truth!). 😁 Muhahahahaha
Just another guy passing by... The unknown tech way...