We believe that there was to different Gavin Dalziel (Dalyzel) and I will set out our reasons for believing this.
After many years of research on our Dalziel line and in discussion with others researching this family we believe that Gavin Dalziel (born 17 March 1722) who married Martha Fisher in 1745 and had the following children;
Janet 5 Jan 1744
Gavin 4 Dec 1748
Gavin Dalyzel who had the following children with Margaret Lenox (Lennox), no marriage found.
Mungo 4 Dec 1747
Agnes 28 May 1749
Margaret 22 Mar 1751
Elizabeth 23 Aug 1763
The dates of the children's births overlap and the parish records (Sorn) record the births as either Gavin Dalziel & Martha Fisher or Gavin Dalyzel & Margaret (Margret)Lenox.
I trust this explanation is of assistance.