Question: What exactly did Del and Rodney's mother die of?
Answer: Joan Mavis Trotter (Nee Hollins) was an unseen character in Only Fools And Horses, having died in 1964, 17 years before the series began. She died of an alcohol related illness due to both her sham marriage to Reg Trotter and being hit hard by the death of her secret lover Freddie "The Frog" Robdal.
Chosen answer: An apparent long string of illnesses.