Just a small postscript on this: I'm grateful for all the help on this, but I've been completely unable to access the bridged HG612 from behind the router. I can connect a PC directly to the unused ethernet port, but this of course bypasses the firewall, so it isn't a usable solution. Port binding doesn't help - in fact I've realised that port binding is a restriction, not an enabling feature (when a port is bound to the WAN connection it can't be used for anything else).
So for the time being I've given up on this approach, which is a pity because in all other respects the combination of a bridged HG612 and a wireless router is exactly what I would like to use. It works very well, but I do need the monitoring capability.