Put all but 2 strawberries into a pan along with 100ml water and the sugar. Over a medium heat, cook strawberries until soft enough to mash, about 3 mins. Take off the heat and squash the berries, using a fork, until pulpy. Add the marshmallows, then stir them into the hot strawberries until they dissolve. Leave to cool.
Whip the cream until it holds its shape. Fold the cream into the cooled strawberry mix, then spoon into one bowl or separate pots and chill for about 2 hrs, or until set. Cut the reserved strawberries in half and use to decorate.