Sorry I have been unable to find any additional information. Did William Allen have a brother Stephen? If so This may be the family in the 1851 census.
ALLEN, Stephen Head Married M 72 1779 Farmer
 Pewsey, Wiltshire, England   
ALLEN, Anna Wife Married F 68 1783 
 Bourton, Wiltshire, England    
LEWIS, Mary Anna Visitor Married F 30 1821 Wife
 Pewsey, Wiltshire, England   
ALLEN, Stephen Hy Son W M 29 1822 Farmers Son
 Pewsey, Wiltshire, England    
KIMBER, Ann Servant Unmarried F 19 1832 House Servant
 Pewsey, Wiltshire, England 
 
I do not know of a Bourton - Anna's place of birth, in Wiltshire or the meaning of the W which is the "Condition" column for Stephen jnr. Widower perhaps?