"The £ is printed just fine on some parts of the receipt!"

That's probably a hint that it isn't the printers fault.

I would guess that some other system that is used to enter what's available on the menu is using CP 437 and somewhere an encoding step (CP 437 to Unicode) is missing so we get the ú character.

I wonder what character we would get if it was a "5€ cocktail" instead.