Preheat the oven to 180°C (160°C fan) / 350°F. Grease a baking sheet or shallow ovenproof dish with Butter and line with baking parchment.
In a large bowl, combine Self-Raising Flour, Caster Sugar and a good pinch of Salt.
Add the Suet and rub it into the dry ingredients with your fingertips until the mixture resembles coarse breadcrumbs.
Stir in enough Whole Milk to bring the mixture together into a soft, slightly sticky dough.
Turn the dough onto a lightly floured surface and roll it into a rectangle about 25 x 30 cm (10 x 12 in) and roughly 5 mm thick.
Spread the rectangle evenly with Raspberry Jam, leaving a 1 cm border all the way around.
Brush the border with a little extra Whole Milk, then roll the dough up from the long edge into a tight log. Trim the ends and tuck them under so the roll sits seam-side down.
Place the roll on the prepared parchment, brush the top with a little melted Butter, then wrap tightly in the parchment and then in foil.
Sit the wrapped roly-poly in a roasting tin, pour in boiling water to come halfway up the sides of the parcel, and bake for 45–60 minutes until the pudding is cooked through and springs back when lightly pressed.
Carefully unwrap the pudding, dust with a little extra Caster Sugar if you like, slice and serve hot with plenty of warm Custard.
