Thanks for the replies. We are likely to go to see both again and will speak to staff.
Our feeling is Hampton may be the more academic of the two but would probably suit better a more confident boy. We like the space there, compared to KGS, and the array of clubs and activities - which he would like - is amazing.
We are drawn to the co-ed, and possibly gentler, aspect of KGS. It's strong maths, which DS likes (so is Hampton) and also had a decent clutch of clubs. And a little smaller.
We liked Reeds, too, though too expensive for us and too far (we wouldn't have seen much of DS who would be last stop on long bus ride).
Instinctively, probably leaning towards KGS (or Tiffin) but wishing there was a bit more space. DS not into team sports but loves outdoors, climbing etc.
DS himself, admirably, doesn't seem to care. Asked what he liked most, he said the toilets at Tiffin. Guess that will clinch it.