There's a known issue reported on the portal:
We are aware of issues with the presentation of incorrect solar data on Energy graphs on the web portal and app and are actively working on a fix.
My 12x425W panels were shown in the daily report as generating over 6kW at peak output a couple of days ago. It was definitely suspect because the graph was very "peaky", whereas the day concerned had been uninterrupted sunshine. I ran the report for that day again yesterday and the graph was much smoother, with a much more sensible peak output, so it looks like they've taken steps to fix the problem.
In summary: Unfortunately, it looks like it was a glitch in GivEnergy's software rather than a surprise PV bonus.