That's all really interesting and also what I discovered when looking through the A & D registers too, so obviously we're probably both on to something. It would make sense that 'our' Mary Ann only appears in the workhouse records from 1883 because in her examination she states that William paid her a maintenance for two years. In fact, there seems to be one entry which I identified as might being her dating from 24 April 1882 - 18 Jan 1883 at Lambeth Workhouse. If William cut her off because he needed the extra money to move out of the Peabody flats with Rosetta (which he did in July of that year) this would be about the right time frame.
This is true, and William and Rosetta had an extra mouth to feed by July 1883.
One thing though; there is mention in the settlement record that Mary Ann went straight to Lambeth workhouse from Peabody buildings when the couple separated. If the separation was in 1881 then that may indicate that she is the woman with an 1840's birth year (which is correct) and not the woman giving a birth year of 1850's who enters Lambeth workhouse after 1883 ( the birth year also given in workhouse records we know definitely relate to 'our' Polly Nichols).
The Lambeth Creed Register shows the woman born in the 1840's was in the workhouse for the 1881 census on April 1st 1881, even though the census does not corroborate this as I mentioned but instead shows an older woman of the same name, whom the Creed Regsiter shows wasn't there on census night.
"..Married at St Brides Church Fleet St 16 January 1864. Maiden name Walker. Husband has got the certificate-When we separated I went
direct into Lambeth workhouse from No 6 D Block. Then it was agreed husband should allow me s 5/- a week which he did for 2 years
which I received from Mr Tavener at Lambeth Workhouse..."