Table: england_counties_1891_1961
Columns: administrative_county
area_statute_acres_1891
population_1891
area_statute_acres_1961
population_1961
headquarters

Question: What is the Population n1891 of the County with Sleaford Headquarters?
SQL: SELECT MAX("population_1891") FROM england_counties_1891_1961 WHERE "headquarters" = 'sleaford'