My advice would be Fuji Acros instead of HP5 for this kind of shots. Hands down better for this in my opinion based on far superior reciprocity characteristics.
These might give you a sense of my results for something you might be after (tech specs for development in the image titles, most taken at f/5.6 for between 3-5 minutes):
http://www.flickr.com/photos/38698047@N00/222902921
http://www.flickr.com/photos/38698047@N00/222902863
http://www.flickr.com/photos/38698047@N00/412281169
http://www.flickr.com/photos/38698047@N00/222903028
All taken late at night.
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