Hi
M&S seem to do this a lot for their beer range ... getting breweries to brew their (better) beers for them, under contract, changing them just enough to be classed as "different" (probably for the breweries' sake) but not so much as to not still be familiar
... e.g. their "Cornish IPA" is/was made for them by St. Austell, and was a scaled down (ABV-wise) version of Proper Job, the Northern Mild is/was a stronger version of Thwaites' Nutty Black (I don't shop there often, but those two used to even be bottle conditioned, like the brewery originals, with the brewery primary strains
), the Norfolk Bitter is a variation on Woodforde's Wherry, their London Porter is based on Meantime's London Porter, the Southwold Bitter is something like Adnams' Southwold, etc., etc.
My guess is that they are slightly different, probably so as to not "compete" with the originals ... but possibly also to fit better to M&S customers' "requirements"
Cheers, PhilB