In the 1851 census at Shortmead Street:
William Twelvetrees H M 29 Carpenters journeyman & stationer. born Biggleswade
Emma M 23 Dressmaker b London
Henry 4 son London
Edward 6/12 son Biggleswade

He is also mentioned in Slaters directory 1850. William Twelvetrees printer & stationer

There is an obvious connection between all the Twelvetree families living in Biggleswade as this William was also connected with carpentry.  Was this William Twelvetrees the brother of Harper, he was a year older?
There is a tithe award plan of 1838, but only William Twelvetrees living in Stratton Street is mentioned. The Stratton Street location has been involved in the building trade all these years but, now a stonemasons yard with some of the old wooden barns still extant.