Toads with a flat batter-y
Good Yorkshire pudding is not an art, it's a science. You need three things:


Hot fat
A hot oven
Batter of the right consistency


The only raising agent in Yorkshire Pudding is steam from the water in the liquid ingredients. You need to convert that water into steam fast to get a good rise, and you need the batter to crisp up quickly so it doesn't collapse.

This is why you need hot fat and a hot oven. I would say 180C is much too low: use 220C*. Preheating the dish is a good idea - use a good, thick, metal pan, preheated along with the oven. Then place that on the hob and fry the sausages in it, then add the batter. Then it's straight in the oven. This ensures as little heat as possible is lost . Do not open the oven during cooking if you can possibly help it. The sudden drop in temperature and the shock of the door closing will destroy the structure.

Adding cold oil to the pan is, as you can imagine, a bad idea. Instead, fry your sausages with enough oil to rise the batter with (if you're worried about the fat, you shouldn't be eating Toad in the Hole anyway) and it will get good and hot while you fry the bangers. If it doesn't sizzle when you pour the batter in, you might as well throw it out.

Batter consistency is the hardest thing to get right. Too thin and there won't be enough gluten to provide structure. Too thick and it will collapse under its own weight. About the consistency of double cream is right, but experience is the only real teacher. I usually whisk my batter with a balloon whisk, leave it to sit for a couple of hours, then give it a good whisk before adding to the pan.

*Oven dials lie. Get an oven thermometer to see how much. My last oven was 30 degrees cooler than it said on the dial.