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.
Chart showing the number of references in each month of the diary’s entries.