District One mansion, the marble and large-format tiling needs attention, how do we get it done to the right standard?

You're right to treat this as specialist work, marble and large-format are unforgiving and expose a general tiler immediately. Done to standard means: a properly prepared, flat, stable substrate (big slabs telegraph any deviation as lippage or hollow spots), the right setting method for large format, full adhesive coverage with back-buttering so there are no voids, and natural stone sealed correctly, unsealed marble stains and etches. For repairs, stone has to be matched for colour and veining, which is genuine skill, a mismatched slab is worse than the original fault. What to insist on: no hollow tiles (tap-test), no lippage across big formats, correctly sealed stone, and matched replacements, not just 'close'. At this level the 'fine from three metres' standard doesn't apply, it should be right up close. A stone and tiling specialist used to District One mansions will work to that. Insist on flat, solid, sealed and matched, and hold them to it.