Preheat your oven to 350°F (175°C) and boil a kettle of water. Grease a large piece of parchment paper and a similarly sized piece of aluminum foil with a little butter.

In a large mixing bowl, whisk together the self-rising flour and salt. Stir in the shredded suet and grated chilled butter until evenly distributed throughout the flour.

Gradually pour in the whole milk, using a butter knife to stir the mixture until it starts to clump together. Use your hands to gently knead it into a soft, slightly sticky dough, adding a splash more milk if it feels too dry.

Turn the dough out onto a lightly floured surface and roll it into an 8x12-inch rectangle, about 1/2 inch thick. Spread the raspberry jam evenly over the dough, leaving a 1-inch border around all the edges.

Starting from the short end, gently roll the dough up tightly like a Swiss roll. Pinch the ends firmly together to seal the jam inside and prevent it from leaking during cooking.

Place the rolled pastry seam-side down onto the buttered parchment paper. Brush the top lightly with 1 tablespoon of milk and sprinkle evenly with the granulated sugar.

Wrap the parchment paper loosely around the roly-poly to allow room for it to expand, then wrap the aluminum foil over the parchment, sealing the edges tightly to create a watertight parcel.

Place the foil-wrapped pudding into a roasting pan and carefully pour the boiling water into the pan until it reaches halfway up the sides of the pudding. Bake in the preheated oven for 1 hour and 30 minutes.

Carefully remove the roasting pan from the oven and lift out the foil parcel. Let it rest for 5 minutes before unwrapping, then slice into thick rounds and serve immediately with hot vanilla custard.