The most important thing is to make sure your fat is really hot before you put the batter in.
I cook the sausages on 180 for a bit, and then turn the oven up to 225, and make sure the fat sizzles when you start to pour the batter in.
I actually don't measure out my batter (I have been making Yorkshire puddings about once a fortnight for about 15 years), I put about 5oz flour in a mixing bowl, and then add about 2 eggs until all the flour is absorbed. If it takes 3 then so be it.
then add the milk a drop at a time, making sure it is properly combined before you put any more in. When you have enough milk in it is the consistency of double cream.
1. Preheat the oven to 220C. Heat half the fat in a frying pan over a medium heat and brown the sausages on all sides.
2. Meanwhile put the eggs in a large bowl and beat, preferably with an electric hand whisk, until thick. Beat in the flour and milk alternately until smooth, then stir in the water and mustard and leave to sit for 15 minutes.
3. Put the remaining fat in a roasting tin (about 25cm x 20cm) and put in the oven to heat. Once the sausages are browned all over, and the batter has rested, take it out of the oven and put over a medium flame. Pour in the fat from the sausage pan, followed by the batter, which should sizzle as it hits the tin. Add the sausages and return to the oven.
4. Bake for about 35 minutes until well risen and golden, then serve immediately
I've found that it makes a difference if you use a metal pan rather than something like Pryrex - it transmits the heat better and consequently the batter rises better. Having said that, my latest toad (as pictured) was a bit 'all sides and no middle' but it was still delicious