Beat the eggs with an electric hand mixer until they are foamy and thick. Add the flour, milk and ale bit by bit, alternating between flour and liquid, while still beating on a low speed, until everything is incorporated.
Add the mustard and stir through. Leave the batter to sit for at least half an hour.Preheat the oven to 200C/190C fan*/gas mark 6.
Put half the beef dripping in a roasting tin (the one I use has a base measurement of 29x23cm) and melt it on the hob. Add the sausages, turn them over in the fat then transfer to the oven to roast for 10 minutes.
Add the leeks, turn them over in the fat along with the sausages again, and roast for another 10 minutes.
Remove the leeks and sausages from the roasting tin. Turn the oven up to 220C/210C fan*/gas mark 7. Put the rest of the dripping into the tin and, when the oven is hot, heat this until smoking.
Carefully remove the tin from the oven. Lay the leeks in it, pour the batter on top then add the sausages. Return to the oven for 20 minutes.
Take the roasting tin out, sprinkle on the cheese and return to the oven for 5 minutes. The batter should be puffed up and golden. Enjoy piping hot.