Could Mary have married more than once? Though I have just had a quick look on the freebmd and have to admit I couldn't find another marriage
Have you looked at the censuses before the 1861 to see if you can find William and Mary living together as youngsters before Mary gets married? I'm sorry but I don't have access to the censuses so haven't been able to check. It may not turn up anything but always worth a try.
I am going to post on Lincolnshire as maybe someone near Grantham will have access to PR's etc seeing as they married in Grantham & were on census too.
I had thought she may have married previously also was thinking she may have married after William died as she has 2 more children but looking for his death I find 2 entries not sure which one is her William
Dec1865 7a p.289 or Mar 1874 7a 302
it does however make me think if there are 2 William Christopher Huthwaites then I need to check ALL my fact again
will post link on here if I get any results from Lincs board