Contours not displaying on local mapnik rendering

I have a Ubuntu 11.10 VM running the Ubuntu Tile Server as described on the wiki. I've imported the Geofabrik Oxfordshire extract and the usual OSM data is rendering well. I cannot, however, get any contour lines to display.

I have followed the wiki'd instructions covering the PostGIS Approach, using the import script provided to add the following files to my database:

• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N51E115.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N51E117.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N51E118.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N50E115.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N50E116.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N50E117.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N52E115.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N52E116.hgt.zip
• http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Eurasia/N52E117.hgt.zip

These files, I imagined, would correspond my existing OSM data.

To /etc/mapnik-osm-data/osm.xml I have added the line:

&layer-contours;

To /etc/mapnik-osm-data/inc/layers.xml.inc I have added the line:

!ENTITY layer-contours SYSTEM "layer-contours.xml.inc

(note, I've removed some tags as the formatting was making my line disappear. This line is formatted the same as the others)

The file /etc/mapnik-osm-data/inc/layer-contours.xml.inc is as pasted here

I restarted renderd and got the same permissions error as described here. Since granting the gis user select on geometry_columns the error has been removed, but I still cannot see any contours.

I have connected to the database remotely to verify that there is data in the contours table.

I've not done anything with extents here - are they required? Anything else I'm missing?

Help greatly appreciated.

Thanks, Joseph