Add the caster sugar and cream into a medium sized saucepan (the mixture will bubble up!) over low heat. Stir regularly until the sugar has completely melted into the cream.
Allow the mixture to bubble for 2 minutes (no more! I set a timer), stirring constantly. Keep the heat on low and make sure you are stirring thoroughly so that none of the mixture can stick to the bottom of the pan.
Remove from the heat and stir in the lemon juice. Allow to cool a little while you prepare the base.
Add the digestives or graham crackers to a food processor and blitz until resembling fine breadcrumbs. You can also place them in a food safe bag and bash with a rolling pin until crumbled.
Mix in the melted butter until fully combined - the mixture will resemble damp sand. Then press the mixture into a 9 inch tart case making sure the bottom and sides are equally covered.
Scatter half of the raspberries (100g) over the bottom of the tart case, then pour the lemon mixture carefully over the top. I like to use a fork to press the raspberries down so they are all fully covered by the lemon mixture.
Place in the fridge to cool and set for a minimum of 4 hours, ideally overnight.
To serve, decorate with the remaining raspberries and any other fruit you like!