Mix the flour, suet and salt with enough cold water to form a soft, slightly sticky dough. Roll out two-thirds into a circle big enough to line a 1.2 litre buttered pudding basin, and the remaining third into a lid.
Line the basin with the pastry, pressing out any cracks with floured fingers. Prick the lemon all over with a skewer or cocktail stick, right through to the middle, so it can release its juice as it cooks.
Pile the butter cubes and half the sugar into the basin, sit the lemon on top and pack the rest of the sugar around it.
Wet the pastry edges and press the lid on top, sealing it well so nothing can escape during the long steam. Cover with a pleated sheet of foil and tie with string.
Steam for 3 hours 30 minutes in a pan of simmering water, keeping the water halfway up the basin and topping it up with boiling water as needed.
Rest for 5 minutes, run a knife around the edge and turn the pudding out onto a deep plate. The lemon and buttery sugar melt into a pond of sauce that floods out around the sponge. Serve in bowls.