QuoteBTW: do you understand why this way http://www.openstreetmap.org/browse/way/34844315 has layer=-1 ?? (from point of view of OSM data)I'm sure that's because it goes under the A5 - but I don't think that's the right way to do that.
However, as far as I could see the problems with Milton Keynes were all connected with Layer = -1. If the park or residential area are not rendered, the roads (and stream) are all there, so they are not under the blue polygon.