I thought the question asked in the thread title was How old was the song entitled "Twa Recruitin' Sergeants"? If so where does a song and variants of "A Recruiting Sergeant" come into it? Or indeed the "Over the Hills and Far Away" references.

As far as I am aware at no point in Queen Victoria's long reign were British troops ever raised to march through France or Spain.

Militia Regiments were raised specifically for Home Defence and joining one was a means of dodging dangerous service overseas with the regular army.

In 1739 however the Black Watch changed from being a "Militia" Regiment to being a regular infantry line regiment so they could be sent wherever they were required.

During the French Revolutionary and Napoleonic Wars some 40 "Militia" and Regular Line Regiments were raised in Scotland. All the lowland Scottish Regiments had to recruit - not the case with most of the Highland Regiments who were privately raised as basically single clan regiments, notable exceptions being the Black Watch and the 71st Glasgow Highland Regiment. For example the 79th Regiment of Foot (Cameron Highlanders) privately raised and paid for by Sir Allan Cameron of Erracht if the Regiment needed men then the Head of the Clan just sent his "Tacksmen" out with orders to gather the number required. A relatively small clan they did expand their base for recruits in the design of their tartan which was a combination of Cameron and Macdonald (Sir Allan Cameron's wife designed it and she was a Macdonald).

Specific mention of the Black Watch and the line "Through Gibraltar to France and tae Spain" means that it had to date from a period after the Regiment came into being and from a time when Britain was at war with both France and Spain and a time when it was not present in Portugal.

The line "And a' they've listed is forty and twa" is merely a device to weave the number of the Regiment into the text of the song.