Right. I think I've now got this chap nailed.
I'll post this for posterity, just so that anyone else who may come along might find it useful.
Also, there may be others who can fill any small gaps ...
Baptism: 26 August 1785: Scotlands People index has only one baptism in Banff 1780-1790 for an Alexander Ross- 26/08/1785 to William Ross & Isabel Garden; as he names his first daughter Isabel and a later son William I think it safe to say that this is our boy.
Military: (from circa 13 Feb 1804 to 1 July 1819): From Alexander Ross's entry in the Register of Ordnance Pensioners in 1819 it is possible to piece together his military career. By that time he had served for a total of 15 years 137 days, which suggests that he signed up around 13th February 1804, but I can't establish where.
By June 1812 Ross was serving as a gunner in - I believe - the 6th Company 2nd Battalion Royal Artillery under the command of Thomas Paterson; I am connecting him with the 6th Company as that is one of only two Battalions to be based in Leith in 1812 (the other being the 1st Company) but from November 1806 through until August 1811 the 6th spent a great deal of time in Ipswich, Suffolk, where Ross's son William Swanson Ross, was later to live. Of course, I could be wrong ... He was definitely stationed in Leith Fort, Edinburgh as indicated by his marriage to Jane Swanson in the July of 1812 and here the 6th Company remained until being disbanded in Dublin, Ireland on 31st January 1819, although Ross had moved on before this.
Using the Pensions Register again, it would seem that, having served for 11 years and 46 days in the 2nd Battalion, he was transferred to the 9th Battalion Royal Artillery on or about 1st April 1815 and here he remained until his entry in the Register of Ordnance Pensioners (where there is a reference to his previous service with Artillery, which must be the 2nd Battn.). He was discharged on 1st July 1819, aged 34 and suffering with a pectoral complaint, which seems to have been quite a common ailment amongst the soldiers. He had served for 4 years and 91 days in the 9th and still never risen above the rank of Private.
(More details as to where and when the 2nd Battalion RA went can be found here -> http://www.napoleon-series.org/military/organization/Britain/Artillery/c_2ndArtyBn.html)Occupation: (from circa 1804 to 1863): By the time of his registration as an Ordnance Pensioner in 1819, Alexander Ross was a labourer; in the 1841 census he was an agricultural labourer, but by 1851 he was a Chelsea Pensioner and by 1857 he was recorded as a dyer on son William's marriage to Rachel Stanfield.
General: In his registration as an Ordnance Pensioner in 1819, Alexander Ross was described as being 5' 7" tall, with light hair, grey eyes and being of fair complexion. He was also aged 34, giving him a birthdate of c1785 in Raffen (Rathven), Buckie, Banff.
Marriage 5 June 1812: It is highly probable that the marriage ceremony took place at 293 Canongate rather than at the Canongate Kirk. Apparently, until the late 19th century, it was the norm for a wedding to be held in the bride's home and weddings in the church itself were very unusual indeed. Certified (witnessed?) by John Lindsay, Gunner in said Batn. and William Buchanan, Private in the Renfrew Militia.
Death: 26th May 1863: Bogshalloch, Rathven, Banff, Scotland. From the Banffshire Journal and General Advertiser of that date:
"SUDDEN DEATH. - An aged man, named Alexander Ross, residing at Bogshalloch, Enzie, had (says a correspondent) gone an errand to a neighbour, and immediately after his return dropped down and expired. He had walked about a mile and a half."MT