Only one man named Denis Breakwell served overseas with the British Army in WW1. When he first went (on 14 July 1915) he was serving as Private 3187 of the Army Cyclist Corps. He later served with the Duke of Wellington's (West Riding Regiment) and then the Royal Defence Corps. This info is from his medal index card, which you can find at Ancestry or at the National Archives 'Discovery' website. Sadly it does not seem that his service record has survived but he has entries in the rolls of the 1914-15 Star and the British War and Victory Medals.