Heavy, stodgy, overly sweet - the badly-made jam roly poly served at my school was everything doctors warn us against. But this one is actually quite light, not too sweet, and utterly wonderful.
Make the Dough
In a large bowl, mix the dry ingredients together, then mix in the milk. Tip the dough out onto a floured worktop, then wrap it in plastic film and put it in the fridge to rest for half an hour. Roll out the dough to a longish rectangle, then spread the jam on it, leaving a 2cm (1 inch) margin all round.
Assemble the Jam Roly Poly
Preheat your oven to 180°C (356°F) if it's a fan oven, 200°C (392°F) if not. Butter one side of a sheet of greaseproof paper. Roll up the dough, moisten the open edge with water and gently press down with your finger to seal it. Roll the roly poly into the centre of your greaseproof paper, making sure the seam is on the underside. Raise the paper over the dough and fold it over to form a seal, but leave plenty of room for the dough to expand. Twist the ends like a Christmas cracker and tie them off with string. Now wrap the whole thing in foil, again leaving plenty of room for expansion.
Bake the Jam Roly Poly
Place a wire rack over a roasting tin and half fill the tin with boiling water. Put the pudding parcel on top, and bake for 35 minutes. Remove from the oven and allow to cool for about 5 minutes. Unwrap the pudding, cut into slices about 2cm (an inch) thick and serve with custard.