Hello Jim, it was you who cracked the mystery of which Division he was attached to and the Brigade he was with - that was a couple of years ago.
He was with the Honourable Artillery Company, 2/B, which arrived in France June 1917 (from memory now). The HAC had become the 126 Bde by that point (which you'd found). From various War Diaries concerning the 48th Div, it looks like they were moved around to whichever operations needed their artillery backup. So it was only a few days before the start of the Passchendaele campaign that the 126 and 291 Bdes were allocated to their post.
A problem with war Diaries is that the ones I've been sped-read through do not specify the 126 Bd per se - it feels a bit like the HAC men are independent operatives who don't have a good handle on record keeping! (Lt. Colonel Clifford was their boss). I've pieced together a bit from here and a bit from there to form the picture of where they ended up on the battle front. So I may have erred, but I think I've got it more or less right.
The 3 diaries where I've found info in are: 48th (SM) DIVISION (GS) (Diary, Orders, Reports, Maps etc), 48th DIVISION A & Q BRANCH (Diary, Casualties, Honours List), 143rd INFANTRY BDE (Diary, Orders, Operations, Maps, Narratives, Report Qs, Report Answers). I looked at a few others but they were self-inflicted red herrings.
The filters I used for the Diaries were St Julien and October 1917, especially the 4th. But I've also looked at movements in the month before and after to see what was happening and who was where when. Interestingly, I suspect my Grandfather was at, or near, the same location as his cousin Alec, serving in the Mobile Veterinary Corps. (That's another whole lot research to do one day - my Grandfather is even harder to trace).
I'm attaching the orders which defined the groupings of the attack and also the available artillery. You posted me this originally

It's part of the 48th South Midlands Diary.