Place the raspberries, sugar and lemon in a small pan and warm over a medium heat to bring to a simmer. Stir and mash the fruit as it cooks so that the fruit all breaks up like a sauce. Cook for a couple more minutes then remove from the heat.
Strain the raspberry mixture through a relatively fine strainer to remove most, if not all, of the seeds. Make sure you press through the fruit well to get as much of the solids coming through as you can. Discard the seeds. Chill the puree so that it is at least room temperature if not cooler.
Meanwhile, finely chop the chocolate into small chunks. If some is more shredded, that's fine.
Separately, whip the cream until it forms medium-firm peaks.
Mix the raspberry puree with the sweetened condensed milk. Add this mixture and the chocolate chunks into the cream and fold it carefully so that it is combined (no white streaks) but without loosing too much air.
Pour the mixture into a plastic container or loaf tin, cover either with a lid or with a layer of parchment over the surface and transfer to the freezer. Leave to freeze for around four hours for soft serve, or ideally overnight.