Thanks for the reply Dido. I don't mean to go on about this but in the 1871 census I see Isabella – born Washington aged 9 – with Arabella – born Seaham aged 4 – in the same family. That's why I thought that Arabella became Isabella Mason, since the latter also has Seaham as her birthplace. This doesn't fit with your mention of Isabella dying as a baby, or of Arabella being born in Washington. Perhaps you were speaking from memory?
By the way, just in case you haven't spotted it, in that same census, next door to the 'Dotchin' family, are Joseph and Margaret Gibson and family. She is Margaret Dodgson the sister of Arabella's father John 'Dotchin'. In the 1861 census they are all in the same house (of Edward and Elizabeth Dodgson) at Usworth: the Dodgsons, the Gibsons and the 'Dotchins' (also the Dawsons), although John himself isn't there, just Sarah and their 3 children. Sixteen people in what I imagine was not a large house.
Anyway, I promise I'll leave this to you now.