Keeping records for 56 days

Im sure i read on here from those in the know that as of jan you had to keep your tacho records for 56 days instead of 28 but all i can find online about it is it only applies if you do international work.

im asking as my tacho card ran out and i have been carrying it around with me since the begining of december and being a dense git i keep putting the wrong one in the tacho head and need to hoof it out

Currently it is for Euro trips only.
The rule may be extended to UK use as well.
https://www.rha.uk.net/news/news/detail/new-56-day-tacho-rules-an-faq

Just scribble on the front (away from the chip) in marker pen on the old one to save mixing them up.

As Franglais said though it’s not yet a requirement here, but I wouldn’t trust any self funded agency to play by the rules, so keep it for a while yet.