They are available on Findmypast with a subscription or credits.
It is not as easy to use as it used to be.
Apart from the names previously mentioned, other baptisms of a William and Elizabeth are:
Henry 1828 - abode Bethesda
William 1830 - abode Bethesda
William 1832 - abode Bethesda
It would mean that the two Williams would have died before 1835
There is also a Sarah born to William Parry and Elizabeth Williams, a spinster, in 1828.
St Ann's is close to Bethesda, so the above could be correct.
These map links might be useful to see where they lived:-
http://maps.nls.uk/view/101606301http://maps.nls.uk/view/101606277David