As you already have most of the jigsaw, the PRO should be a good place. At the entrance 3 or 4 months ago, was the description of a day in a WW1 battle - from the war diary of a battalion. The diaries had to be kept on a daily basis, so, that is what you need to see. As you are delving into very valuable documents, you have to have a reader's card and you will have to book into the reading room and reserve the documents.
The regimental museum should be able to tell you when the battalion was formed, but that information might also be gleaned from the Army List for that period. Be warned, at the height of WW1, the annual Army list was published monthly!
I doubt that the battalion was "in training" for that period. It was probably just based in that location. My GF joined up in Mar 1915 and was in France 5 weeks later. Some training programme!!!