Having taken advice, and with some more research, a clearer scenario emerges. Gnr Melvin, in all probability, went overseas with what was originally entitled 1/1st Lowland Field Artillery Brigade, having been embodied with the rest of the Territorial Force in September 1914. The brigade arrived in France on 24 October 1915 (the date on his medal card) joining 51st (Highland) Division in November.
The brigade became 257 Brigade on 15 May 1916 changing yet again to 260 Brigade in the period 3 - 15 Jun 1916. On 1 Feb 1917 the brigade was disbanded and left 51st Division with A Battery becoming C Battery 315 Army Field Artillery Brigade, B Battery becoming C Battery 86 Brigade and D Battery becoming B Battery 84 Brigade (C Battery 260 Brigade had left, split in two sections, in January 1917).
In all of this, the brigade had 3 batteries, later four. In all probability Melvin served with 1/1 City of Edinburgh Battery which became A Battery 257 Brigade (A/257) , then A/260, then C/315.
Without his service record though, we cannot be sure which battery he served with, or even if he left the brigade in its various disguises, which means saying where he was would be difficult.
Complicate but no unusual for the RFA which underwent a number of fairly drastic re-organisations in the course of the war.
MaxD