When I used SQL identifier, it stopped working. The command line gets interpreted as following:
insert into stats select "1" as id, 'count of nulls in "UNIQUE_REFERENCE_NUMBER"' as checks, count("""UNIQUE_REFERENCE_NUMBER""") from points_of_interest."pointx_v2_National_Coverage_Sep21" where """UNIQUE_REFERENCE_NUMBER""" is null
By using the correct type specification at each variable insertion you can get this to work.
Either do trial-and-error or actually reason through what is happening at each position and why it is either correct or wrong (build up the query in parts if that makes things easier).