I would think very slim. Every year, a number of people get a place on appeal. You can find out how many by emailing the school. As those children increase the number of allowed children at the school, or PAN, those in the waiting list cannot obtain a place until the school has gone below PAN which is 180. So, if 10 children win an appeal, there would be 190 children at the school, say 11 give up their place reducing the number to 179 children, then child number 1 would be offered a place. In your case, there are a lot of children who would have to turn the place down.
Could you ask the school what your chances would be? I know there is a lot of movement at other schools, but I have no idea about Dartford Grammar. I just know that a lot of people appeal, 80 or 90.
Thanks for your reply. I am expecting lot of rejections from those 65 before DS. Anyone who gave kent exam, they sent a post asking for their preferences. This is in addition to people who gave their preferences in online system. I am guessing many just gave Dartford as a choice without any serious intention of going there. Last year the cut off was 369 for OOC and DS got 360.