I must admit this is a confusing one
Thanks for the help Trish, I'm pleased it's not just me. Having typed this out I think I might have worked it out as I've been doing it (for the past hour).
According to the 1901 Census
Edward J Blair (b1865) and Margaret AT Denham (b1866) are married.
Margaret A Blair, b1889 is a daughter
Isabel Blair, b1891 is a daughter
Daisy E Blair, b 1899 is a daughter
Robertina Blair, b 1893 is an other relative
Lawrence Blair, b1896 is an other relative
In 1911 Lawrence is listed as their son, they have another daughter, Elizabeth and it says they have had 7 children, 1 of whom has died. Robertina is married so has left home now.
Their father James has swapped his birth place from Gosforth to South Shields (where they are now living) and all the others have swapped from North Shields to South Shields as well.
The 1891 Census you referred to had Margaret and 2 daughters living with her parents. Edward was a coal trimmer so could it be as simple as he was away at sea and she was back with mam and dad?
That just leaves the puzzle of why her maiden name was Denham but her mam and dad are Cameron.
The 1911 census may also be answering the original question of why they married in South Shields - as JenB suggested - one of them was living there for a time.
Michael