Hi Sheila
Once the records came available on Findmypast (for men discharged up to 31st December 1913) about six years ago they have not been available at the NA. But they do provide free access to both Findmypast and Ancestry. Although your local library may also provide the same facility.
On FindMyPast his time as a Regular will be listed as WO97 Series and the final years in the Special Reserve (but also known on FindMyPast as Militia Attestation Papers) are listed as WO96.
Robertson's 3/ number will be when he was in the Special Reserve - they were usually the 3rd Battalion. as a Regular (in the 1st or 2nd Bn) he would have had a different number.
They did not keep the records of men who died pre WW1 while still serving, as WO97 are pension records.
If you do go to Kew then check out the set of Army Lists they have on the first floor. These will tell you where the bulk of each battalion were stationed. Army Records will only say Home or India etc. You do not need a readers ticket to look at these.
More so for the next two you can look at the muster books in WO12/16. These will tell you month by month where each man was. He may have been on detachment and not at the main station. Also from the late 1860s they list the married establishment - approved families, giving name of wife and ages of each child. They also give details of promotions/demotions/when on furlo/if fined for drunkenness, plus remittances sent to families (very rare). Some of which is not listed on his service record. The first and last entry in the musters give the most information.
You need a readers ticket to look at the musters. As will anyone wishing to visit the Reading Room with you. It takes about 30-40 minutes to get the books. So you can look at the Army Lists while you are waiting. You can only order 21 items per day. So get there early, and maybe only look at musters every 2/3 year. The service record will tell you in which battalion he was serving. The 60th Rifles usually had four.
Ken