Preheat the oven to 200C/400F/Gas 6. Butter a large sheet of greaseproof paper and set aside.
Stir the flour, suet, sugar and salt in a large bowl until well combined. Slowly stir in the milk or water to form a soft, spongy dough.
Tip the dough out onto a floured surface and knead for a few minutes. Roll the dough out to a 22cm x 32cm/8Â½in x 13in rectangle.
Spread the jam onto the dough, leaving a 1.5cm/Â½in border. Gently roll the dough up from the short end and transfer to the greaseproof paper, seam-side down. Wrap the roly poly in the greaseproof paper, making a long pleat in the parchment 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 process with a large piece of aluminium foil.
Place the pudding onto a roasting rack set on a roasting tin. Pour boiling water into the roasting tin until the half full.
Bake in the oven for 30-35 minutes.
Remove the pudding from the oven, unwrap the aluminium foil, then snip the string and unwrap the paper.
The pudding should be well risen and lightly browned in places. Donâ€™t worry if the jam has made its way through to the outside of the pudding a little â€“ it will taste all the more delicious.
Put on a board or serving plate and cut into thick slices. Serve with lots of hot custard or cream.