First of all I'd like to thank you for your help. As I wrote in my reply to Frances, after 40 years I actually believe I'm getting there!!
I think the William who married Catherine is the one christened 1748 to Thomas and Jane Q.
As he had to obtain his father's permission to marry in 1768. That would make him 20...so about right.
In 1751 Thomas had a son, Robert christened at K.M. Register states that Thomas is 'Of Bark'. When William married Catherine he is also 'Of Bark'. So this fits. I need to see if he has a brother Richard, mentioned in the William Kelly 1815 Will...but will look tonight. So many thanks to you and Frances...I still can't believe we, collectively, have, as they say 'cracked it!. Will phone my 81 year old Uncle Thomas Kelly and tell him.