Army Enlistment 1783 - 7 Wales

[Dean1]

I am researching a chap who enlisted into the Army age 18 (supposedly, although his dob is in question) at Newtown, Montgomeryshire, Wales on 3rd January, 1787 (dates from old document written by youngest son of the family's military service which has recently come my way).    He is supposed to have joined the 83rd foot regiment on the above date but it had been disbanded at that time.   He possibly joined a militia (I suspect the Royal South Lincolnshire Regiment because I think I found him in Stamford, Lincs in 1795 - 1800 and I know/think he had been to Ireland in the mid-1790s).   What I would like to know but do not have a clue how to find out is, which Regiment/Regiments/Militia were based in Newtown, Montgomeryshire in 1783-7.Grateful for any help or pointers.   Sue