Hi Julie,
I suppose you have already looked for deaths for William and Isabella after 1861? They may have fallen on hard times and thought it best to have Jane adopted? I wonder if the Blythes were relatives. What does it say on the census regarding their relationship to Isabella?
With such a distinctive name, it's not likely that there are two Jane Thompson Croziers (but stranger things have happened), so you've probably found the right one with the Blythes.
As William would be about 24 on the 51 census, it's likely he's not at home, and he may be lodging somewhere - so it may be difficult to identify him.
If you get the marriage certificate, hopefully you'll get his father's name. (Bet it's William!

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Depending on the information you require, at some stage it might be worthwhile asking for a Parish Register lookup for William's birth in Skelton on the Cumberland board. Might be best to do this after you find out his father's name from his marriage certificate.
I'll have another look around for him later today.