For the pastry, mix the flour, salt and suet together in a bowl. Add 250ml/9fl oz cold water and stir together until the mixture comes together as a dough. Knead briefly, then wrap in cling film and chill in the fridge for one hour.
Butter a 1.5litre/2½pint pudding basin. Remove a generous quarter of the dough to make a lid for the pudding and set aside in the fridge. Roll out the remaining pastry on a floured work surface to a 1cm/½in thickness
Drape the pastry into the pudding basin, pushing it down against the base and sides of the basin, and making sure it comes about 1cm/½in over the top. Chill the pudding basin in the fridge.
Heat half of the oil in a large saucepan and fry the ox cheek, in batches, until golden-brown all over. Remove from the pan and set aside. Deglaze the pan with a little water, scraping up any browned bits from the bottom of the pan, then pour it over the ox cheek pieces.
Add the remaining oil to the pan and fry the kidneys until golden-brown all over, then remove from the pan and add to the ox cheek pieces.
Add the onion and celery to the pan and fry over a low heat for 20 minutes, or until softened. Add the chopped garlic, thyme, bay leaves, mushroom ketchup and Worcestershire sauce and stir together until well combined, then stir in the flour. Add the beer to the pan, bring to the boil, then continue to cook until the volume has reduced by half. Add the beef stock, ox cheek and kidneys and stir until well combined.
Remove the pan from the heat and set aside to cool, then chill in the fridge.
Spoon the cold steak and kidney mixture into the pastry-lined pudding basin. Roll the remaining pastry into a circle just large enough to sit on top of the pudding dish, brush the rim of the pastry in the basin with water and place the lid on top. Trim the edges neatly then press together firmly to seal.
Cover the dish with a large circle of baking parchment, with a pleat in the middle to allow for expansion, then cover the parchment with a circle of pleated aluminium foil. Press down tightly around the edges and secure with string.
Place the pudding onto an upturned saucer or small trivet in a large, deep saucepan and add enough boiling water to come halfway up the sides of the basin. Cover the pan with a tight-fitting lid and place on the stove over a low-medium heat, so that the water is simmering gently. Steam the pudding for 4 hours, topping up the water every now and then.
When the pudding is cooked, turn off the heat, carefully lift the basin from the water and leave the pudding to stand for five minutes.
Remove the foil and paper and run a blunt-ended knife around the inside of the pudding basin to loosen the sides, then invert it onto a deep serving plate.