Add the plain flour and eggs into a mixing bowl and whisk until combined. Gradually pour in the milk, whisking continuously until you have a smooth batter. Add the mayo and whisk that in too. (The mayo helps to stabilise the batter.)
Leave the batter to rest in the fridge for 20 minutes.
Meanwhile preheat the oven to 200C.
Use the back of a knife to stretch the rashers of streaky bacon so that they cover the whole sausage. Wrap each chipolata with a rasher of bacon.
Pop the sausages in the oven for ten minutes just to get some colour on them, they don’t need to be fully cooked.
At the same time, pour the oil into an oven proof dish and get that into the oven too until it’s really hot.
Once almost smoking hot, bring it out of the oven and pour in the batter. Lay the partially cooked sausages on top, but away from the edges so the batter has a chance to rise. Sprinkle over the chopped sage.
Bake it for half an hour, without opening the oven door. Once its golden and puffed up, enjoy served with gravy.