As no-one has answered yet... and though I never was a regular (other sites in the Lea Valley took priority).
In my experience, you'll need to be very quick if you want to photograph them there. It might be easier now, if they hang around after the breeding season.
With patience and a good lens you can get some pleasing kingfisher shots at Rye Meads from the Kingfisher Hide. However, you're looking down on them so not as good as eye level shots. There are strategic feeding posts set up infront of the hide so you could, with permission, use the downstairs of the hide which is for wheelchair birders. Hope that helps,