I know what it is but can't find it anywhere on the web at the moment. It was a tune called 'The B Side' by, I think, Fleetwood Mac. The lyrics went "She comes and goes like the wind and rain, Travelling lady's on the move again. Stayed a while to light my night and then she slipped right out of sight ..." Only they just used the bass guitar and no lyrics for the original adverts back in the 70s/80s. Wish I could find it - was a great piece of music.
I believe the tune was 'Travelling Lady', by Manfred Mann and Mike Hugg. It was specially recorded for the ad I think, with lead guitar rumoured to be Jimmy Page. The song, with a slightly different arrangement, appears on the album Manfred Mann's Chapter 3.
I presume you meant the original. The Flake song was composed by UK jingle writer Ronnie Bond, who also composed "Tasty tasty very very tasty" for Bran Flakes, and "I'd rather have a bowl of Coco Pops" for Coco Pops, Kwik Fit and more. Also recording artist . Same. Same name as a famous drummer. Not him.