Diana,
Have you noticed the Orders of Removal record for Margaret TURPIN on Ancestry dated 1860. It gives you some information about her that you may not have. Looking at the 1851 census record for William & Margaret, you may find that neither were born in Welwyn. The enumerator has put ditto for born in Herts from the previous household but not under the place name of Welwyn.
Had you also noticed that there is a William GARDNER, aged 10 born in the County, shown as living in the same house as William & Margaret TURPIN but a different household in the 1841 census. Could he be related to Margaret in some way. Also one of the witnesses at William & Margaret's marriage in 1817 was a Mary GARDNER - perhaps another relative?
Janet