Yes, I know it's an instrumental of Clean Bandit's "Rather Be"! But can I find it anywhere? Can I heck! The M&S website links me to the album track, which I already have.
There are about a thousand remixes and cover versions on iTunes, mainly with vocals, altho' I thought I'd found the instrumental version by 'Magno Matic' or someone - coincidentally, it's since been taken down before I had a chance to purchase...
Also coincidentally - the You Tube clip of the ad/music is now refusing to stream on my computer... I suspect a plot!!!