Make the batter - 140g flour, 2 large eggs, salt pepper and half teaspoon mustard powder, whisk in milk until it’s smooth - then, and this is the important bit, leave in the fridge for at least an hour.
Put 10 sausages and a good glob of light olive oil in a roasting tray for 15 mins at about 190, add the batter then cook for about another 30 mins. You can check after 20 mins, it won’t sink.
Sorry it’s not more accurate but it’s been a trial and error thing. Resting the batter in the fridge seems to trump all the other advice for me though.