I started with VectorMapDistrict, which is a dataset provided as ESRI Shapefiles. It is provided as a large number of shapefiles, so setting up a mapnik stylesheet manually would have been a pain. Instead I separated the styles into an XML file, but added the layers using a python program which scans down the directory tree to add the various shapefiles to the map. The python code is vmdmap.py and the stylesheet is styles.xml.
Then I tried Meridian2. This is much simpler - just one shapefile for each sort of feature (A-Road, river etc.). I kept the same structure with the styles defined in an XML file, and the layers in python, but it could have all been done in a single XML file this time. The python code is md2map.py and the stylesheet is styles_md2.xml.
You can see the results below.
The most notable things are that Meridian2 is much simpler geometry, but it includes more road names than Vector Map District. The OpenStreetMap rendering is fancier because I used the standard OSM style, rather than the home made ones I used for the OS Data. I do like the rocks that appear in the VectorMap District rendering though - I will have to import it into OSM!.