Is it possible that Catherine didn't die in that time frame, ie that they separated and Mr Brutnell didn't state that he had been married before when he remarried Ms Cooper? ( I'm assuming that you have the1923 marriage cert and it doesn't say he had been married before)
Have you also located Catherine on the 1911 census ie pre marriage to see if the family had any other names associated with them that might give you any other clues. ( I'm assuming you have the 1914 marriage cert to check for spelling of names, fathers name etc. )