Complex sentence: Gudmundsson 2005, pp.52--53 By now only three British cavalry regiments, were not serving on the Western Front, but had remained in India on internal security duties, they were the 7th Hussars, 14th Hussars and the 21st Lancers.
Split into simpler sentences: Gudmundsson 2005, pp.52--53 By now only three British cavalry regiments were not serving on the Western Front, having remained in India on internal security duties. These were the 7th Hussars, 14th Hussars and the 21st Lancers.