I agree with Geoff, it's probably a pastiche composed especially for the advert. It starts out like a speeded-up morris tune and ends up very like the Trumpet Hornpipe (Pugwash). It's not just one instrument, there's clearly a drum, or drum machine, and maybe something else going on as well as the melodeon (which is what the squeezebox sounds like to me).

I often wonder whether the advert-makers realise that if they make the music too interesting, people like me are too busy listening to the music to hear what's being said over it.

Marje