The marriage entry for William and Janet was indeed at Provan in 1957 - his name is shown as William Isaac S. Leitch.
William Isaac S. Leitch's death entry is shown as:
William Isaac S. Leitch born 10/9/1936, died Oct. 2002 Carmarthen.
He appears to have remarried in Sept.qtr.1963 Swansea to a lady surnamed Giles (cannot give full name as she could still be living). They appear to have had 3 children in Swansea, a son Mar.qtr.1964, a daughter Mar.qtr.1965 and another son in Jun.qtr.1975 - cannot name as no doubt still living).
There is another marriage shown for him (luckily he has a distinctive name) in June 1996 in Llanelli.
If it is his ancestry you are trying to find then the best thing to do would be to obtain a copy of his marriage certificate (to Janet) in 1957. This was in Scotland and you can order it by registering with Scotland People website. Scottish certificates give far more information than English ones and the marriage certificate will list the names of his parents.
I also looked for his birth on SP and it seems that his second and 3rd. names had been switched around by the time he married. Birth entry is shown as William Scott I. Leitch born 1936 Shettleston - again you can order birth certificate on SP. Cost of each certificate is £12.
Once you get further back in time you can purchase credits instead and actually view copies of the relevant records. However, one step at a time.
If, however, you are wanting to trace any living relatives then I'm afraid we can't help much as Rootschat is for help with tracing ancestors and not living people.
Annette