Every dual rate meter switches at a different time. It’s a bit frustrating, but it’s necessary for the industry to regulate the grid. If everyone switched to night rate at the same time, there would be a massive surge at the switch to night rate every night which would be pretty problematic.
Instead, every meter has a schedule, and it will switch rates 30 minutes before or after this scheduled time. It’s the same discrepancy every day, so even though it’s a bit arbitrary, once you’ve worked it out it’ll be the same every day.
I’ve looked into the industry records for you and your meters schedule for night rate is 00:30am - 7:30am. During British Summer time it will be 1:30am - 8:30am.
The best way to find out how much it varies by is to do a kettle test. Boiling the kettle takes a pretty large amount of energy in a short time, so it’s perfect for working out which of your meter’s registers are currently measuring. So, from midnight, try boiling the kettle every 10 minutes or so and note down when it switches to night rate.
This is unfortunately just one of the small faffs of the energy industry.