By Ruby Flanagan Mary Berry adds one surprise extra ingredient to her mashed potatoes and Brits love it. Dame Mary Berry is one of the UK’s most beloved bakers and food writers and during her six-decade-long career has shared a vast array of recipes – both sweet and savoury – which are enjoyed by many. Mashed potatoes are the king of comfort food – whether it be smoothed withmilk or cream, roughly mashed so you get the odd solid bit of soft potato, or even with cheese added. What’s so brilliant about mashed spuds is that they are incredibly simple to make with the classic ingredients being pot…