Depends what sort of pattern is on the artex. If it's a low profile, gentle pattern, then a medium pile roller sleeve should suffice. For the longer profile type artex, then a long pile sleeve should be ideal. Screwfix sell them. If it is the longer profile artex, then I usually get a dry short pile sleeve on a pole and swipe across all the ceiling to take the heads off the longer areas of pattern. Then I paint it with a long sleeve roller.