If your're going to be running Mapnik under Widows, I may be able to help. Spent several hours getting it to run a fortnight ago. It wasn't easy as all the Unix instructions don't translate over to Windows very well, the conversion template for OSM.XML didn't capitalise entries correctly, causing lots of unhelpful 'node map not found' errors. Nothing on the internet about it. I eventually sussed that that the xml file had different capitalisation for a few things (and some were right) also it seemed to be randomly using double and single quotes, where only doubles would work. I would have loved to have documented it all, but it took several days of on and off fiddling and trying things to get it working.

I'm now very pleased with the results, and can privately render things like tracks with dotted borders, overlayed with any right-of-way info.

I agree! I've been watching the 'designation' proposed tag -- it does look like it'll be generally accepted, and certainly makes sense from a UK perspective. Time to update my previous footpath entries!

Does the designation tag actually affect the rendering? I've been following the tagging guidelines at:
osm.wiki/UK_public_rights_of_way