I really wanted to see if I could download the music from the current Guinness ad 'The World', so I emailed Guinness to see exactly who it's by.
They told me it's a cover version of Mason William's track 'Classical Gas', which we already knew, but that it's actually a version released by a Nashville group called Area Code 615, recorded in the 1970s.
Here's the recording up on YouTube:
http://www.youtube.com/watch?v=3ynnSdBIda8 (synced to a video of birds for some reason...)