Yes I've got the thames to mapnik render all the way from slough to the estuary by adding the natural=water tag in addition to the waterway=riverbank tags on the pieces of river. However, I've been having problems with Potlatch and the map loading times which caused spurious ways to be added to the map. This has caused all of the Belvedere area to appear to be covered with water. I've now corrected this problem with JOSM, but unfortunately for this week, the mapnik map is stuck with this ugly mistake - I just wish there was a way of forcing that mapnik tile to be updated.