I too have been looking for the name of that particular piece of music, so far without success. However, I am fairly sure that it wasn't written specifically for the "Flake" advert owing to the fact that my brother had an album in the eighties with old love songs on it and this was one of them. It had lyrics etc and was a beautiful song. It took me completely by surprise as I too had thought that it was nothing more than a "jingle" up to that point. I am sorry that I can't be more helpful than that but it has to be out there somewhere and somebody knows what it is called and who the artist was. Theres a chance, I suppose, that it was converted from a "jingle" into a proper song after it became famous from the advert but all I know is that either before or after "Flake", it was indeed a real song.
Regards, Mick.