Preheat the oven to 200°C/400°F. Butter a large sheet of baking parchment and set aside.
Stir the flour, suet, sugar and salt in a large bowl until fully combined. Slowly stir in the milk or water to form a soft, spongey dough.
Tip the dough out onto a floured surface and knead for a few minutes. Roll the dough out into a 22 x 32cm rectangle.
Spread the jam onto the dough, leaving a 1.5cm border around the edge. Slightly dampen the border with water. Gently roll the dough up from the short end and transfer to the baking parchment, seam-side down. Wrap the roly poly in the baking parchment, making a long pleat in the paper to allow the pudding to expand as it cooks. Twist the ends of the parchment like a Christmas cracker and tie tightly with kitchen string, to seal the pudding inside. Repeat the wrapping process with a large piece of kitchen foil.
Place the pudding on the roasting rack set inside the roasting tin. Pour boiling water halfway up the roasting tin, just to the base of the pudding, and cook in the oven for 30–35 minutes.
Remove the pudding from the oven, unwrap the kitchen foil, then snip the string and unwrap the parchment.
The pudding should be well risen and lightly browned in places. Don’t worry if the jam has made its way through to the outside of the pudding and ends up on your face.
Place on a warmed serving plate and cut into thick slices. Serve with lots of custard.