Thank you all so much for the helpful and enlightening comments.
I now think that Mary reverted to her maiden name of Riley after Edward died. (I have learnt it may be an Irish thing!)
There are plenty of deaths of Edward Dawson between 1841 and 1851 so I will go with the thought that Mary went back to her maiden name.
I do have lots of follow ups of Mary Riley from 1861 onwards - all living in the same area.
Cannot find the son William who was 5 on the 1841 census though. I reckon given her name of Mary Dawson in 1841 and Mary Riley in 1851 that Edward died between these years and, as Groom says, he was probably deceased in 1843 when their daughter, Margaret, married Thomas- even if the wedding certificate does not say he is deceased. (Thanks for that Groom)
I cant thank you all enough. I think your wonderful. Cyber greetings !!
