on a 106/saxo mileage is stored only in the clocks.
It's also completely legal to fit a set showing any mileage you want. Mine currently show 3,400 miles as that's what my car has done since a full body/engine rebuild and I bought a brand new set of clocks
The only thing that wouldn't be legal would be trying to sell the car claiming the mileage was accurate when you'd had it adjusted to something else. That's a no-no. Simply having different mileage clocks is fine - as long as you tell the buyer