FreeBMD has an entry which could be worth following up:
Death Mar qtr 1918, Mary Cresswell, age 26, Aston vol6d p554
There are two only two marriages on FreeBMD for Leonard Cresswell between 1903 and 1918 but I am, not sure either of them is your Leonard:
Dec qtr 1906, Basford (Notts/Derbys) vol7b p459, - on same page Mary Ann HORSLEY
Mar qtr 1909, Droxford (Hampshire) vol2c p199, - on same page Mary ROUGH
Good luck!
Philip