surely you could take the tap out and look at the filter easy enough? what is it 2 screws?
may surprise you what you find! I found all sorts of what looked like limescale in a road bike one once, obviously it`s all down too the quality of your local petrol station. Supermarket branches are cheaper because they buy any crap from whoever is cheapest at the time whereas I`ve seen reports of Shell garages coming out tops year in