Hello Marie,
He collected large deliveries of coal (and presumably in his case - stone) from the big central railway stations in a very large waggon. This would be stored in his yard or went straight for delivery to his customers. Everyone needed coal for heating, gas was produced from coal and there was constant building and roadmaking in London, so there would have been a lot of call for stone too. Some merchants were very big time with lots of waggons and maybe branches out in the suburbs too - others were just small time, one waggon affairs.
Here is an extract for "The Horseworld of London" describing the loading of coal in the 1890s - a bit later than your interest, but I suspect nothing much had changed.
"(The horses) go in dozens up that curious thoroughfare - though it looks like a cul-de-sac - which runs out of Pancras Rd. under the arches by Battle Bridge, round by the gasworks and between the Midland and Great Northern Railways. There you will find coals to the left of you, coals to the right of you, volleying and thundering. In every arch is a platform; on every platform are two weighing machines; over each weighing machine is a shoot which delivers into the sacks and from which the coal stream is cut off with a lever, much as you turn off water with a tap. Overhead are the waggons; down the shoots the coal roars and booms, and hisses in a cloud of dust as sack after sack fills up and is run out on the hand truck into the vans, in the shafts of which stand the horses gently bobbing their nosebags and utterly indifferent to the dust and din.
The horses had to be really strong as they sometimes had to pull about three tons.
