Remove and grease your slow cooker pot.
In a bowl, mix the flour, caster sugar, baking powder, lemon zest and a pinch of salt.
Add the butter and mix.
Whisk together the eggs, milk, lemon juice in a jug, then pour this onto the dry ingredients and mix till smooth.
Stir in the berries, then pour the mixture into the slow cooker pot.
In a jug, mix together 250 ml boiling water with the flesh from the lemons and leave to infuse for five minutes.
Strain out the lemon, keeping the water, then mix in the 100 g of caster sugar and stir until dissolved.
Pour this mixture over the batter in the pot.
Cover and cook on high for approx 2 hours, or until the sponge is firm and no liquid areas remain. It took just over two and a half hours on high to cook in my slow cooker, which is quite slow.