Marriage of man and deceased wife's sister not legally possible in E&W before 1907 - See Notes below.
The law in England & Wales did not permit marriage to a deceased wife's sister. Until the passing of the Deceased Wife's Sisters Act of 1907
(see: http://www.litencyc.com/php/stopics.php?rec=true&UID=1430), the Consanguinity and Affinity restrictions of the 1537 & 1835 Marriage Acts prohibited such marriage.
Wickcroft Farm, Englefield still exists, seemingly much as it was during the period of residence of Jenkin and Martha and their family, although the modern farm machinery and the newly introduced "Wickcroft Farm Shop" website woud not be familar to Jenkin and Martha!
See: http://www.wickcroftfarmshop.co.uk/