From the L&M Companion:
The Steelyard. On the s. side of (Upper) Thames St, now covered by Cannon St Station. Once the London house of the Hanse merchants. The property extended to the river, its four acres including the merchants' hall, wharves, warehouses, and private dwelling houses, and in earlier centuries had been an important factor in the life of the city.
More at the 1911 Britannica article:
It's number 22 on this 1666 map:
Cumgranissalis • Link
added to LH entree 1746 location addition :
The Steelyard, Steleyard, or Stilliard, the hall of the Hanse merchants, stood in Upper Thames Street, where the Cannon Street station now stands. The superficial area of the place amounted to four acres. The principal entrance in Thames Street was formed by stone gateways.
The Steelyard, from the Middle Low German Stalhof / Dutch Staalhof, was the main trading base (kontor) of the Hanseatic League in London during 15th and 16th centuries....The Steelyard, like other Hansa stations, was a separate walled community with its own warehouses on the river, its own weighing house, chapel, counting houses and residential quarters....In 1475 the Hanseatic League finally purchased the London site outright and it became universally known as the Steelyard, but this was the last outstanding success of the Hansa. In exchange for the privileges the German merchants had to maintain Bishopsgate, one of the originally seven gates of the city, from where the roads led to their interests in Boston and Lynn.
Chart showing the number of references in each month of the diary’s entries.