Hi Andrea,
yeah I know... welcome to my research world

unfortunately I don't have any of Grandad's sources... I suspect that back in the 60's he did most of it by visiting the local records offices and the like.
As far as I can see, the connection is the eldest daughter on the 1891/1901 census for the Jennison family: Hannah Mary born in Durham Stockton 1887/8. I have searched FreeBMD and the GRO Indices available through Ancestry and I can't find a birth for Hannah Mary Jennison, however there is a birth for Hannah Mary Sellers Q4 1887 Stockton Vol. 10a Page 56. Which might explain "Jane's" change of forenames. Incidentally, the male Jennison children mostly carry Jennison family names, of the girls, there is this Hannah Mary and a Lillian Jane.
Also, the most likely candidate for the birth of Mary/Jane/Ann that I can find is this one: Mary Jane Seller Q1 1865 Whitby Vol. 9d Page 392... The Whitby Reg. District includes Egton, which, phonetically, isn't a million miles away from the "Eckton" recorded on the 1901 census as her birthplace.
I have looked at the 1901 James Sellars, in 1881 that family is living in Attercliffe and I can't find any likely place links as yet.
Naturally, I can't trace this Henry Seller on the 1861 census either... can you see him at all? in 1851 he appears to be at HO107 2375 37 2, the son of John and Hannah Sellars and born in Sleights. His mother Hannah was born in Egton....
I have tried GenUKI for the various place names, most seem to be in the area of Guisborough & Whitby, as far as "Jane" goes I can't find Ormshead but Ormesby includes the village of Eston (Eckton?), also Easington (Eckington?) Parish includes Boulby. Skelton in Cleveland has Boosbeck, no joy with Lamps Land, Harbourn Dale or Rayth, though Whitby parish has Raithwaite and Iburndale. Whitby also includes Sleights (1851 birthplace of Henry Sellars).
I haven't found Lidnham in Norfolk either making Henry's wife Hannah no easier to trace...
I think you're right on the certificates, it may be my only other source of information to help clear all this up... or muddy the waters even further
Thanks for your help, I'm hoping that this family will be the one I am looking for
Ian