Just to add a few more bits of data to the discussion;
H DEXTER # 78,156 enlisted in the RFC on 01/01/17 -- From a copy of RAF muster roll on Asplin military pages.
The birth of Marion Nelson S(mith?) DEXTER is indexed on Scotlands People for Leith South, in 1918
Herbert DEXTER lace warehouseman in NTT in 1911, is 35 yo. Is it likely that he would enlist in the RAF in 1917? There are two younger Herberts both in LIN in 1911 (Grantham and Louth born 1897 and 1892 respectively) who might be better fits.
Rick