Is this your William's parents marriage....
In 1875 a Frederick William Hollis married Sarah Shirley...
Where was your William Hollis born?
On the Free BMD site this is the only Frederick Hollis who married a Sarah.
Where was your William born?
Where has he been found on the 1901, 1891, 1881 census years? Have you looked for him?
From what I can gather no Hollis are to be found in this area in the 19th century ???which contradicts that Frederick Hollis was a native of the Wrexham area.
The Hollis name was most popular in the Potteries & Derbyshire.
This is a very confusing matter... You have the wedding certificate... Locate the chap in 1901, 1891, 1881 & info may jump out now that several possible leads have been put forward.