From 2400' in the average light single you cannot glide clear of the built up area if the Lea Valley, as quoted, "is still regarded as being a congested area of a town, city or settlement". Presumably the same rule would apply to the river. The text quotes Rule 5 as being the need to "alight clear of the area" (not just pick a little spot to land within it) which would rather suggest that the debate is over - you can't do it according to that leaflet. Not quite sure why the leaflet wouldn't go as far as to explicitly say that. Perhaps because if you happened to be in a motor glider you could make it? Any other ideas?