The interesting bit is at Sainsbury's you can mix 6 bottles and get another 25% off on top of discounted price making this £12 -> £9 for a whole 1L (equivalent to £6.75 per 75cl so this is cheap as normally at £12 full price everywhere).
Sainsburys single bottle discount is the 1 litre bottle which is not as cheap by volume as the Asda rollback offer of £8 for 75cl (reduced from about £11).
But if you want a single 0.75cl bottle, £8, then Asda rollback deal offer better and link is here:-https://groceries.asda.com/product/port/taylors-first-estate-reserve-port/43102
(Not sure if Tesco has this too on offer at £8.)
And this is far better value than the M&S Special Reserve 0.5L , even when discounted at 50% off from that ridiculous high price, which is also the same ruby made by Taylor's if you don't want the M&S decanter glass bottle.