Wow, I posted a reply last night and had an early night. I was so surprised to see so many incredibly helpful replies this morning. I would like to state that although I've been researching my family tree for a couple of years, I'm essentially a beginner. Also in my original post, because I was asking about the surname change, I didn't disclose everything I knew about Daniel Gavin/Giverin.
Heywood, thanks for the prisoner info. I think you are right, the older Daniel Gavin is my relative. The person who told me that my relative was in prison for attempted murder obviously got confused with the younger guy. Doh! You are spot on about John and Annie Watts as the wedding witnesses. They are indeed Daniel's sister and brother in law. Why didn't I twig that? I was only researching those two a couple of days ago.
Ruskie, I don't know when Daniel's parents (Joseph & Mary Ann) died. I think Mary Ann may have died first because in the 1901 census I have a Joseph Gavin aged 62 living alone in Salford. I can't find out what happened to Mary Ann Gavin neé Smith.
Millmoor, thanks for that. I've been in contact with an 82 year old distant relative that I've never met who lives in Canada . He has also been researching the family tree. He is one generation older than me so Daniel was his grandfather. He told me that his own father had spoke about Daniel. He said that Daniels wife's family had arranged to have him institutionalised and that he had ended up in Lancaster. He said he might have had a drink problem but all the evidence now seems to point to some mental illness or other affliction. I had already noted that Daniel had attended St Josephs industrial school aged 12. I'm not sure why he would have been sent there. Perhaps, to use today's terminology, he had special needs.
Indiana, you asked if lunacy runs in the family? My wife is beginning to think so

Once again, thanks to *everyone* who has replied. I only asked why the surname had changed but I received so much new and valuable information about my family tree and I'm very grateful.