I expect you've used search engines like Google, which produced this interesting set of factoids:-
http://www.surnamedb.com/surname.aspx?name=MancktelowLooking in the 1841 - 1901 census for England, the surname shows 9 heads in 1841 (all in Horsemonden, Kent), 6 in 1851, peaks at a headcount of 46 in 1881, and reduces to 39 by 1901.
It seems to be prevalent in the South East, dominated by Kent.
Of course, the census has vagaries and many errors, so dont take the numbers etc for gospel
