Scrape the loose off. Fill it with a wide, flexible, filling knife or a plasterers trowel, not a small, stiff scraper. The best tool to use, if you can get one is a dry-liners metal bladed spatular, which is basically a 10″ wide filling knife. Dont over fill it, press on hard and fill it flush, you want minimal rubbing down. Let it set and repeat on the deep stuff if needed. Best stuff to use is easifil, available from B&Q for about £12. (get a clean bucket to mix it in while you’re there!)
There ya go, piece a p1ss!