[Simon..Hewison]

Surely in 2025 it is within the wit of an IHD to keep its clock right and aligned with GMT/BST!

have you ever seen how complicated the timezone files are? Keeping millions of such embedded system updated with that is very difficult.
The IHD specification for tariffs isn't sufficient for the half-hourly time-of-use tariffs like Agile, so Octopus don't bother trying to keep the tarifff data sent to the smart meter IHD accurate, presumably because they'd have to send millions of tariff messages to the smart meters every half an hour.