You really need to find out if they are proper quarry tiles or the cheaper rougher pavers ,unglazed; which used to wear out quickly and got recoated, almost yearly ie. These may just a natural patina or they have been painted or sealed over the years. They are not really waterproof and if the protection is removed and the original coating (usually candle wax)disturbed they will suck in the damp. Scratch can be repaired with furniture crayons.
You will have to equalize the area of smearing to get it right, . what you are really doing is replacing years of muck.
NT used to do a booklet I f i remember right. Historic Scotland also do one.
Beware though, the origional glaze on many of them was lead based.