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I use the cheap Muji ones and the Muji gel pens (which are refillable for under <£1)

Get through a notepad once a month or so and a refill every 3 months.

Love their pens so much! I always stock up when I'm in a city with a Muji store

The Muji ones cost considerably more than the cheapest similar notebooks from eg a supermarket.

Correct. But they are the minimum "fit for purpose" I have found i.e. don't disintegrate or soak up ink like the cheap supermarket ones.