The Cadbury's Flake jingle was written by my sisters ex.
(He also wrote the coco pops jingle)
The royalties bought a house in Mykonos.
The team responsible for the Smash mashed potato ads mentioned upthread - the ones with the contemptuous aliens - went to see the then go-to-guy for jingles. They had a chat and explained what they were looking for - just a simple note sequence to accompany the closing strapline: For mash get Smash. The guru, sitting on his piano stool at the time, turned to the keyboard, said "Something like this?", and played four notes. The creative director burst out: "That's it!" The guru said "No, I was just saying, for example..." "No!" declared the creative director, "That's it! Write it down. Now." So he did.
Nice work if you can get it.