Sarah Robins Lawday married whom, where & when? but first, as Sarah is on the maternal side of this family........how do you know her surname is Lawday? from a registered marriage cert? could it be Lawsey? it may have been mistranscribed by you/someone (easy to do) and who are the witnesses at the marriage?
And if you have concrete evidence that she was a Lawday (I can see a nice family of Lawsey's with James aged 45-49=45 the father being a tailor

) living in M.E.O.T with a possible mother aged 73 living in Marylebone.....the only Lawsey's in town ie Middx
Start by building families of Lawday's.........their aren't that many in London in 1841/1861
for example......
1861
5 DIGBY STREET, ST MATTHEW BETHNAL GREEN, MIDDX
THOMAS LAWDAY HEAD 38 AWL BLADE MAKER BETHNAL GREEN, MIDDX
MARY " WIFE 31 "
THOMAS " SON 8 "
WILLIAM " SON 6 "
MARY " DAUR 4 "
ELIZA " DAUR 9 MONTHS "
-----------------
1861
17 PUTNEY STREET, ISLINGTON, MIDDX
WILLIAM R LAWDAY VISITOR 40 (MAR) COMMERCIAL TRAVELLER
ST PANCRAS, MIDDX-----------------
1841
HAMPDEN STREET, ST PANCRAS, MIDDX
WILLIAM LAWDEY 35 CHEMIST & DRUGS
NOSUSANNAH " 40 YES
EMMA " 8 YES
THOMAS " 2 YES
-----------------
1861
30 WHITEFRIARS STREET,
ST BRIDES, LONDON, MIDDX
CHARLES LAWDAY HEAD 32 CHIMNEY SWEEP LONDON, MIDDX
(London orphanage Asylum, St John Hackney...Charles Lawdy aged 12 in 1841 census)ELIZA " WIFE 29 "
-----------------
Now you need to find those (above) London born Lawday's in 1851 (not available to me) and then find them in 1841
that may or may not lead you to these..........
THOMAS LAWDAY married SARAH NEILD 17 nov 1808
St Brides (Fleet Street), Middx
JOHN LAWDAY married MARY GOODMAN 13 nov 1811 " "
WILLIAM LAWDAY married ANN BRENNEN 13 mar 1827 " "
JAMES LAWDAY married ELIZABETH PERKS 5 dec 1802 St Anne Soho, Westminster, Middx
Bryant