Grease a 20 x 25-cm/8 x 10-inch ovenproof dish or roasting tin.
Make the batter by sifting the flours and salt into a mixing bowl. Make a well in the centre and add the beaten egg and half the milk. Carefully stir the liquid into the flour until the mixture is smooth. Gradually beat in the remaining milk. Leave to stand for 30 minutes.
Preheat the oven to 220°C/425°F/gas mark 7.
Prick the sausages and place them in a dish. Sprinkle over the oil and cook the sausages in the oven for 10 minutes until they are beginning to colour and the fat has started to run and is sizzling.
Remove from the oven and quickly pour the batter over the sausages. Return to the oven and cook for 35-45 minutes until the toad is well risen and golden brown. Serve immediately.