At the top of the cellar steps the walls are made of hammer dressed Carboniferous limestone. These were deposited in warm shallow water marine conditions similar to the Bahamas today. You can see the remains of organisms which were alive at the time. The fossils are of the phylum Brachipoda, a two shelled organism now nearly extinct. The skeletons are made of CaCO3, calcium carbonate so will dissolve in a weak acid.
The boiler house opposite is roofed with slates which are metamorphic rocks and do not let in the precipitation as they are crystalline and are impervious. Slates were formerly mudstone which have been put under intense pressure and squashed releasing the OH molecule contained within the sheet silicate crystal lattice. The slates are graded with the larger ones closer to the gutter and the smaller ones nearer the apex of the roof. This is an older style of roofing, common in times when money was no object and manpower cut the slates.
Turn right at the top of the stairs and look at the wall on your left.