Are they "posh" sausages ? If so I'd add about a tablespoon of oil, if cheaper ones I'd not add any and pour off some of the fat before adding yorkshire mix. I normally cook for about 10 mins then add mix.
Nigellas is brilliant and mine come out great every time. I haven't got a hob proof roaster so fry the sausage patties and heat up my roaster in the oven then pour the sausages in and add the batter. It comes up huge and tastes great