Ok All
I've had a think about this and lets assume his 1877 wedding cert is correct and he was a widow. That would mean his first marriage would have to have been between the 1861 census, as he is listed as unm rather than wid, and the time Emma Halford got pregnant with Charlotte, ie between mid 1861 and the end of 1865. Furthermore his wife must have died in the same period.
If we also assume the wedding was in Northampton, as he was working there, freebmd lists only two possible John Smith matches. One, in jun qtr 1861, to either Eliza Frost or Harriet Howes (an Eliza Smith d sep 1863 qtr). The other, in jun 1863 qtr, to either Sarah Flavell or Louisa Hewitt (two Louisa Smiths d in the period, dec qtr 1864 & jun qtr 1865).
So my betting would be that my mans first marriage is one of these two John Smiths. Is that a reasonable deduction?
Regards
Anthony