Thanks to everyone who looked earlier.
The Mary Ann Williams saga - the plot thickens.
I have received the death cert of the Mary Ann Burton death in 1880 and it is not the right one. Wife of a William Burton.
I also applied for the 1878 birth certificate of Anna Burton, who was shown on the GRO as MMN Barlow.
And she IS the daughter of Joseph Burton, labourer, and Mary Ann Burton formerly Barlow, of 28 Star Street.
So I am beginning to think that the previous suggestion is right - that Mary Ann WILLIAMS (shown as mother of my grandfather Joseph William Burton on his 1876 birth cert) may actually be the same person as Mary Ann BARLOW.
And I need to have a good look at the very local Barlow family kindly found by jonw65. I haven't yet had the time to sit down and get my head round it all.
Any thoughts as to why in the 1911 Joseph and Mary Ann said they'd been married 31 years (so 1880), with 4 children, 2 living/2 died, which wouldn't have included young Anna, or my grandfather born 1876?
Answering my own question, I wonder if Mary Ann was born Barlow, married a Williams, and then lived with Joseph Burton, and that her Williams husband didn't die till 1880?
Given I can't find any marriage for them, they're scarcely avoiding bigamy, are they!
Added: hadn't realised I'd also got the birth certificate for Mary Ann BARLOW born 1868 St Georges.
We had theorised that she might be the Mary Ann BURTON aged 13 who appears with Joseph Snr, wife Mary Ann, Joseph Jnr, and Elizabeth in the 1881 at 28 Star Street.
The certificate says she is the daughter of George Milton Barlow, and Mary Ann Barlow formerly Williams of 3 Princes Square. George Milton Barlow's occupation is 'Cook at the Sailor's House'
Now totally confused in the Barlow Williams saga.......
Ignore that last section. I think George Milton Barlow is a completely different family. Just coincidence re the Barlow/Williams names.