Seems Mercer and Newland didn't get along with each other. Mercer's troop at Waterloo.
From The Art of Alexander Cavalie Mercer
Second Captain Robert Newland had served in Spain throughout most of the Peninsular war before joining G Troop. There was clearly animosity between Mercer and his more experienced deputy and in his letters written late in life to Henry Leathes, he suggests that Newland left his post for a safer one. Newland had an exemplary record having fought at Salamanca, Vitoria, Orthes and Tarbes and had often been commended and it is not easy now to establish the truth in all this. I have delved into this relationship more in my novel about Waterloo entitled Voices of Thunder published by UPSO in 2003.
The Charge of the Red Lancers on Mercers Troop of Royal Horse Artillery by Chris Collingwood.
Waterloo - 18th June 1815. The Red Lancers made five separate attempts on the allied squares and batteries suffering heavy casualties but failing to break either. Mercers artillery troop would not retire and served their guns continuously regardless of repeated attack.