Put the cranberries in a pan with 150g of the sugar. Gently heat until the sugar begins to melt, then cover and cook for 5-10 mins until the berries are soft. Mash with a potato masher and transfer to a large mixing bowl. Stir in 200ml of the cream and chill until cool.
In a bowl, whip the remaining cream with the remaining 50g sugar until soft peaks form. Scrape onto the cranberry mixture and fold 1-2 times to very lightly marble. Spoon between 5 small bowls – the fool will marble more as you spoon into the dishes. Cover and chill for at least 2 hrs or up to 24 hrs.
Froth the egg white with a fork. Put the extra 3 tbsp sugar on a small plate. Tip the extra cranberries for decoration into the egg white, then drop into the sugar. Shake the plate to roll and coat the berries. Repeat a few times, leaving the sugar to set for a couple of mins between each. Chill until ready to serve.
To serve, top each fool with a few sugared cranberries and serve with Cinnamon shortbread (see 'Goes well with').