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Prepare the cheesecake the day before serving. Leftovers keep for 2-3 days in the fridge.
Preheat the oven to 150°C, fan 130°C, gas 3. Grease and line a 20cm x 30cm traybake tin (not loose-based) with baking paper.
Crush the biscuits into fine crumbs in a food processor, or in a large plastic food bag, using a rolling pin. Mix with the melted butter; press firmly into the base of the tin. Chill while you make the cheesecake layer.
Put the chocolate in a heatproof bowl over a pan of gently simmering water, making sure that the base of the bowl doesn’t touch the water. Leave to melt, then stir until smooth and set aside to cool slightly.
In a food mixer, or in a bowl with an electric hand whisk, mix the soft cheese and caster sugar for a couple of minutes until well blended. Add the eggs, one at a time, followed by the cocoa, soured cream and vanilla, then the melted chocolate. Finally, add the food colouring until tinted to your preferred shade of red.
Pour the cheesecake filling over the chilled biscuit base, then sit the tin inside a large roasting tin and put on the oven shelf. Pour hot water into the roasting tin so it comes about two thirds to three-quarters of the way up the outside of the traybake tin. Bake for 1 hour, or until the cheesecake is set but still has a slight wobble. Turn off the oven, but leave the cheesecake inside to cool for an hour with the door closed.
Remove the cheesecake from its water bath and leave to cool completely. Cover and chill overnight.
For the jelly topping, if adding, blend the raspberries to a purée and press through a sieve, discarding the seeds. Soak the gelatine leaves in cold water for a few minutes. Dissolve the sugar in 100ml water in a small saucepan, then remove from the heat and add the squeezed-out gelatine sheets and stir. Combine with the raspberry purée and lemon juice and pour evenly over the top of the chilled cheesecake. Chill for 2-3 hours until set.