I am in the process of rendering a series of map tiles based on the OS OpenData products using the gdal2tiles.py script (and an updated version that uses all cores on the machine to speed things up). The different raster products are rendered at different scales and then displayed using LeafletJS and OpenLayers applications as simple demonstrations.
The following command generates the tiles for the zoom levels I need:
python gdal2tiles.py -z '7-9' -e -p raster -r average osvmd.vrt osvmdtiles/
This creates zoom levels 7, 8 and 9 using the average resampling method from the osvmd.vrt virtual raster and outputs the tiles in the osvmdtiles directory.
The issues with zoom level 7 appear to be random. It could be anything to do with the source raster, the projection. It is just odd that the same virtual raster works for the other zoom levels and not that one.
If you have any answers, please post them here: http://gis.stackexchange.com/questions/68672/gdal2tiles-py-problem-creating-mini-tiles