Hi Derek,
I can't help you with your question, but I suspect we might have some sort of a connection.
My Great-Great Grandfather, James Smith, had parents Thomas Smith and Helen Parry. They married in early 1837 in Holyhead, unfortunately just missing civil registration, although the marriage is in the IGI.
It is suspected that Parry is not Helen's maiden name (she was born in approx 1803 in Beaumaris). She had a daughter, Matilda Helen Parry, born in approximately 1827. Matilda is shown on the 1851 census incorrectly as Matilda Smith. She went on to marry Jacob Mealing Mills in 1854 in West Derby, Liverpool.
Either Helen had been married before to a Mr Parry, or Matilda was illegitimate. These were actually fairly well-to-do types, that tended to have less illegitimacy.
Now, what leads me to suspect that we have a match is that according to the 1891 census, Matilda was born in Llandsaint, Angelsey. I suspect that this is an alternate spelling of Llanddeusant. (Incidentally, in 1861 her birth place is given as Llanheddi - I can't find anywhere named this and this may be a handwriting issue).
From what I can tell, Llanddeusant looks like a fairly small place, so if there are Parrys in that area, maybe they're related? Maybe your William Parry is Helen Parry's brother? Alternatively, maybe Helen's mysterious first husband is your William Parry's brother? Do you have any thoughts?
Thanks
Darren