12 hour repeated timers sometimes have an hour missing

[birdman]

I did ask here and in Reddit but nobody suggested a better alternative than recording it off Freesat/Freeview. Any ideas?

"All" you need is a process to produce the output stream and pipe it through a process that switches output files every twelve hours.
I suspect it's the sort of thing that is technically fairly simple, but since it's an unusual thing to do, no-one has written anything to do it specifically.