Preheat the oven to 200C/400F/Gas 6.
Sift the flour into a bowl with the salt and add the light brown sugar. Add the suet and just enough water to create a soft, but not sticky, dough.
Place the dough onto a floured work surface and roll out into a rectangle about 20cm/8in x 30cm/12in.
Brush one side of the pastry with the warmed raspberry jam, leaving a 1cm/½in border all around the edge. Fold the jamless border in and brush with milk.
With the short side of the pastry sheet towards you, roll up the pastry loosely and seal the ends well.
Place the pastry roll onto a greased baking sheet with the sealed edge underneath.
Brush the pastry roll all over with beaten egg and sprinkle with demerara sugar.
Place into the oven to bake for 35-40 minutes, or until golden-brown and cooked through.
Remove from the oven and sprinkle on a little more demerara sugar.
To serve, place generous slices of the roly-poly into bowls and serve hot with custard.