Hi,
Just to say that Plants For a Future has got a little
information about ground cover plants http://www.scs.leeds.ac.uk/pfaf/grdcover.html
as well as a checklist of 250 plants which can be used for ground cover. http://www.scs.leeds.ac.uk/pfaf/ground1.html
Although I'm not sure if they are anything like as comprehensive
as the other resources mentioed so far.