Pick up a leaflet in store - there are x4 £5 off a £25 spend , only one can be used per week in January.Or save £10 off a £50 spend -this can be used any date in January.
You will find loads of these leaflets sitting at the checkouts. Can only be used for food,drink,flowers or plants.
I've picked up a few leaflets and plan to use the £10 off of £50 for each week in January. If you do a big shop in M & S you could use two vouchers & get £20 off £100.
Also worth noting is that Tesco will accept these vouchers as long as the spend is £25 or £50.