Put the gelatine sheets in a medium bowl and cover with cold water. Leave to soak for five minutes or until they soften. Turn them a couple of times to make sure they don’t stick together.
Pour the squash, lemon zest and 100ml/3½fl oz of water into a small saucepan and heat very gently until just warm. Remove from the heat.
Squeeze the gelatine sheets to remove excess water and then add to the pan, stirring until it melts in to the liquid.
Stir in 250ml/9fl oz water and the yoghurt until they are thoroughly combined.
Pour the jelly into four glass tumblers or dishes, cover with cling film and chill for 5-6 hours or overnight until set.
Serve the jellies topped with fresh berries and mint leaves to decorate.