Preheat the oven to 190C/375 F/ Gas Mark 5 and place a large deep baking tray carefully filled half way with boiled water from the kettle on the bottom shelf of the oven. I always take the kettle to the oven to avoid a difficult transport of a tray half filled with water. This will give a good amount of steam in the oven which will help the pudding to cook in a wet steamy environment, exactly how it loves to be treated.
Cut a large piece of silver foil 50 cm long and place to one side and also cut another piece of baking paper (not greaseproof paper) about 40 cm and place that on top of the foil for later on.
In a mixing bowl place 200 grams of flour, 50 grams sugar and 100 grams of suet.
Add the tsp vanilla extract and 150 mls of milk to the bowl but keep back a little as you may not need it all or you may actually need a splash extra.
Use a wide bladed knife or metal dessert spoon to mix it into a shaggy dough where it is just coming together into a ball.
Use your clean hands to squish it into a loose ball and tip it onto a floured work surface.
Knead it briefly into a tighter ball. If the dough is too sticky add an extra sprinkle of flour on top of the dough and knead a little more. You want it soft but not so sticky that you can't roll it out.
As soon as you think it is dry enough (not too sticky but still quite soft) to roll out, then flour a rolling pin and roll out to about a 23cm x 33cm rectangle.
Spoon the jam on top of the dough and spread carefully leaving a gap of 2cm all around the dough.
Roll the dough up carefully from the short end, I always want to do this along the longer edge but trust me it is definitely the short end, don't roll it too tight or all the jam will start trying to squish out as you roll and you'll be in a world of jammy pain.
Make sure the seam is sitting under the roll of dough when finished.
Place the roll on the baking paper and roll the roll up in the paper loosely to allow the roly poly to rise and get bigger as it cooks. Twists the ends closed like a boiled sweet in it's wrapping.
Repeat the same with the silver foil.
Place on a large baking tray and bake for 50 minutes
Allow to cool slightly before unwrapping (watch out for the steam in the paper) and slice into jammy swirls and serve. It's not the prettiest centrepiece so I always slice it to show the cute swirls within. I'm sure there's an analogy there somewhere but just pass me the custard.