Different mothers? COMPLETE

[Maiden Stone]

Birth regs. (Lancashire BMD):
1854 Margaret Fielding mmn. Ormshaw
1859 Thomas Fielding  mmn. Fielding   

There was an 1849 baptism of a Robert Ormshaw, son of Ann Ormshaw. No father's name recorded but occupation was farmer. He may have been son of Robert & Ann Ormshaw and be a red herring. Seems to have died as an infant. 
There were a few likely Ann Ormshaw/ Ormshire/ Ormisher  born around late 1820s.
John Fielding may have been a made-up name. 

[Maiden Stone]

Several children born in 1820s, '30s & '40s with a father Robert Ormisher etc., some occupation farmer, some labourer. They include Ann, Martha and Claris.
Another Martha Ormshir (etc.) or possibly more than one of that name, was single mother to several children in 1830s & '40s, in Wigan and environs. Her mother may have been Margaret Ormshir (etc.) who had 3, possibly more "base" children during early decades of 19thC. Baptism entry for her 3rd noted that it was her 3rd illegitimate child. She was definitely a busy lady.

[KGarrad]

Thanks everyone!

The puzzle over different Mother's Maiden Names has been resolved.

Further, as Thomas was only my step-grandfather (my gran's 2nd husband), the Ball family are only a peripheral part of my tree.

Think we can call this complete.

[Maiden Stone]

Marriage of Ann Ormshaw to James Fielding looks to be the correct one. A James Fielding of the right age died in 1850 in Wigan. If that death was Ann's husband then he couldn't have been father of Margaret and Thomas. She may have been a widow when they were born. The name John Fielding as father on marriages certificates of Margaret and Thomas Fielding may have been them remembering the wrong name of a supposed father they'd never known. Alternatively, James Fielding had a brother John.
Ann Houghton's POB was Upholland on 1861 census.
1841 census 11 year-old Ann Ormesher was a servant at The Sportsman in Upholland.