RootsChat.Com

Hi Seemex,
Had a look at  Aberdeen PO directories. Under 1st Aberdeenshire Engineer Volunteers,
1883-4 William M Thomson is a Sgt Instructor
1885-6 & 6-7 He is Sergeant Major
He has an entry in the main directory as 8 Union Wynd (the Unit Office) and home 4 Claremont Street in 1866-7 And 8 Windmill Brae in 1888-9.
The directories are at
http://www.nls.uk/family-history/directories/post-office/index.cfm?place=Aberdeen

I think you have been misled, their Annual Camp was at Maryculter.
In 1881 their Headquarters were at 48 Huntly Street. In the 1882 Standing Orders, contact details for key members are given and there is a Sergeant Instructor Thomson at 53 Huntly Street. The rank indicates that he was Regular RE as the Volunteer Instructors are named as such. Hope that is useful.
My information comes from two old books in the care of the Aberdeen Branch RE Assoc. There were probably earlier versions but they have not survived. Unfortunately, there is no information for the period 1882-90.
I already have a PDF of the Scottish book in the link and tried a OCR of the Aberdeen one, but like the archive version several of the key pages were almost unintelligible.
However, in pursuit of research on the Unit Officers which I am undertaking, I must pay an extended visit to the Aberdeen Central Library where they have a copy of the Aberdeen book.There may also be entries in the Aberdeen Post Office Directories for the relevant years

Obviously a while since you were on this forum, but there is a Chelsea Pensioner William Innes born Keith 1820 which seems about right on the National Archives. The Chelsea Pensioner records are accessed through FindMyPast. This would tie in with him being married in Ireland and working with the Militia in Edinburgh.