The Mission Hall was probably the pitched roof building below the sign. This extract from a Kings Cross Heritage Study (
https://www.kingscross.co.uk/media/48-HBS-Part-1.pdf) explains:
1.1 Culross Buildings constructed 1891-2 by the Great Northern Railway (GNR) as rented accommodation for its workers and others displaced by the enlargement of King’s Cross Station into the Milk Dock area immediately to the south. Long range, of four storeys over a non-residential basement. 40 dwellings in five sections, each with walk- up open-fronted central staircase. Yellow stock brick with red brick bands, pilaster strips, and other decorative features. Flat roofs for clothes-drying and children’s play area.
1.2 Two-storey Culross Hall at the eastern end of the range served as a mission hall, with pitched slated roof. Smaller two-storey block on corner of Battle Bridge Road and Cheney Road, at one time also used as Mission Hall, now with flat roof but possibly originally pitched.
1.3 Basement storey, accessible only from Milk Dock site, housed workshops under Culross Buildings and boiler room under Culross Hall.
This is a photo of part of the building c2004.
Just realised that I have a photo of the same building.
Dave