Combine the strawberries, Cointreau, orange zest and caster sugar in a large bowl, cover and refrigerate for 1 hour. Drain the strawberries, reserving any juices.
Purée about a quarter of the strawberries with the reserved juices in a blender or food processor. Divide the remaining strawberries among four glasses.
Beat the cream and icing sugar until soft peaks form, then fold the strawberry purée through the whipped cream. Spoon the mixture over the top of the strawberries, cover and refrigerate until needed.