I have been using my unlocked HG612 as a modem only on my post office (talk talk non LLU) ADSL connection for several months, and earlier this week after maintanance on the line
http://status.zen.co.uk/broadband/outages.aspx?number=01437 the modem was in sync (it had not even dropped it) but the pppoe connection was unable to connect.
After checking the line could sync if a router was used in pppoa mode I decided to configure the HG612 as a router using the settings shown in the attachments below.
The biggest annoyance is that on the hg612 one cannot set reserved DHCP addresses and also the router I was using, the asus rtn66, this cannot be used as a dhcp server in access point mode, so I had to use my old faithful (still used as a second AP) Linksys WRT54GL as the dhcp server.
Using grc.com's shield up port scanner, I mostly get closed ports not stealth that I would prefer; but browsing seems a little faster and I can still access line stats and configure the modems snrm etc.
Ian