> I want to capture a full Web tree at one point in time, 
> but then subsequently just update different 
> sub-directories of the full tree. 

Err, you can't do that - updating certain areas of the 
site, and not other ones. There is no obvious way to do 
this, unfortunately .. you can force the reloading of data 
files by erasing them and then use 'continue an interrupted 
mirror', but it won't work with html files.