I haven't been to see Ewell Castle myself but I know a couple of boys there and I know parents who have visited it recently.
The ability band is definitely lower than Whitgift/Trinity but Ewell Castle sells itself on having smaller class sizes than those schools (they say classes are about 16). Sport is pretty big and, apparently, the grounds are beautiful.
A friend who visited recently said that the school looked tired and a bit run-down but that they are about to build some new stuff (can't remember the details - sorry!!
The boys I know there are happy - it seems like a good school for a less academic child with sporty tendencies. By less academic I don't mean low ability or anything - I just mean a child in the tier below the high fliers!!
Hope this helps!