Preheat the oven to 180 C / Gas 4. Grease and line a 28cm x 18cm baking tin with greaseproof paper. If using a silicone tin, it is still a good idea to grease and flour it as the cake is very sticky when warm.
Mix the flour, sugar, ginger and bicarbonate of soda together in a bowl. Using your fingers, rub the margarine in until the mixture resembles breadcrumbs. Add the beaten egg, syrup and hot water and mix well with a wooden spoon until combined. The mixture should have the consistency of a medium to heavy batter. Pour the mixture into the prepared tin.
Bake in the middle of the oven until golden-brown and the top is springy to the touch, 35 to 40 minutes. Leave to cool in the tin. Once cool, turn out carefully and cut into nine portions.