Hello Anna
It was quite common for a person to be known by another name & use both for different events.In your case you're fortunate to have the parents living with children indicated on the censuses,so you know Sarah was known as Ann,Sarah being her birth name.
I can find only one marriage for Thomas to Sarah Plant/10/8/1807/Burton - on - Trent/Staffs.Sarah(Ann) states on the census that she was born in Atherstone so is unlikely to be her.
jim