Hi Helen
Re Preston Barracks:
Forgot to mention that I have a submission about the Barracks on another website (My Brighton & Hove, Places, Preston Barracks) If you Google that, the general brief, illustrations and the many supporting comments may be quite sufficient for your needs.
Re Attrestation Papers:
Access to them really depends where in the world you are. When I wanted my great grandfather's details, I was able to actually go to Kew armed just with basic details like full name, date of birth and a location to look him up. These helped me find his army number and from then on, things quickly slotted into place. Fortunately, mine was an unusual name, but be prepared for the false trails left by any other Frederic William Belchers.
So to start, Google "Attestation Papers," and get onto the website of the National Archives and read what they have to say.
Re Frederick's wife
If you found Frederick in the Barracks in 1911, does that census show his wife was with him or nearby, or do you think had she been left elsewhere?
(Note: Although the Barracks was in Preston (Steyning district) by 1911 the rapid urban expansion of Brighton meant that by then, the housing from one parish to the other was continuous)
Roy G