Dan
I am no expert on this but my experience in the past is that if vpi/vci are wrong you can login briefly but the connection is rapidly lost. My scan of the web suggests o2 do not use the default settings hence I suggested looking at the tripod pages.
You have probably already also seen this:
http://www.o2help.co.uk/configure-netgear-router/
Sorry can offer no more advice. I certainly did not need to do anything other than use the default settings with a HH2B and Zen. My experience of non-standard settings was for AOL sometime ago with a hacked HH1.
Peter