Hi Sue
I have found a reference to them having a 2nd Battalion, from 1804 to 1817, who were involved in most engagements from Doura (May 1809) to Toulouse (April 1814), including Salamanca in July 1812. The 1st Battalion were in the Cape and Ceylon.
As you now have a regiment you could go to Kew and look for him in the muster books. Researching a long service using musters is time-consuming, and therefore expensive if you have to pay for a researcher. However you may be able to shorten the effort by taking this period as read and checking the 83rd in (say) 1818 and beyond to get his discharge date. Then look at the 1790s to see if he was in the 83rd when they were reformed - although they appear to have been in the West Indies during the 1790s – but they would have kept a depot in the UK, and maybe they wanted his tailoring skills at headquarters. Or he could have been in a different unit and only joined the 2nd/83rd Regiment later on.
A man’s first entry in the musters of a particular regiment will say if he was a recruit, or give the name of any previous unit. Likewise his final entry will say if he was discharged (and maybe with or without pension), or was transferred to another unit. As you are looking for his parents you could just concentrate on his enlistment. The musters will not give his parents. But if you are lucky his first entry in a muster may give his parish of birth, and age on enlistment. Failing that you should see where the Sergeants were out recruiting.
If his first muster does not give you anything useful, you will have to find his last entry, as again they often give parish of birth etc.
Another option could be to look at the church records in Stamford and Great Baddow looking for other military BMDs. One may give a regiment, or with an officer’s name you can usually find the regiment from army lists and the London Gazette.
Ken