Preheat the oven to 170°C. Place a large, deep baking tray half full of water into the bottom of the oven.
Cut the butter up into cubes and mix with the flour. Rub until it is a fine crumb consistency. Add the suet and mix well.
Add the milk a bit at a time, mixing to form a dough. Add a bit more if you need it.
Place onto a work surface and roll out into a square (about 25cm x 25cm)
Tear off a piece of foil about 30cm x 40cm and place on a work surface. Tear off a piece of baking parchment paper about the same size and place on top of the foil.
Spread the jam on top of the dough square, leaving a 5cm gap along one side.
Roll up towards the jam free side. Seal the pastry, brushing with some water if necessary.
Carefully transfer the roll onto the centre of the greaseproof paper (maybe using a baking tray).
Wrap up the pudding in the foil/paper, scrunching up, but leaving quite loose as it will get bigger on cooking.
Transfer the roll into the oven, placing above the baking tray. Cook for an hour.
Remove and cool for a few minutes before opening the parcel. Slice and serve with custard.