Thanks for that information,Stuart. To be honest, I'm getting more than a little frustrated at the inconsistency and variability of the traffic data reported by routers. I've a few reports now of the same router having different interfaces in different situations (the HG622 being one such example). I've now switched back to using the HG612, and I find that the interface which delivered the correct data the last time I used it (atm1) now fails to deliver, and the ppp256 interface gives proper information. The value reported by atm1 has stuck at 4294967295 bytes (2^32 - 1).
I don't think I'm going to spend any more time on this feature, as it seems to work properly once you find the appropriate interface, and there's little or nothing I can do to automate that. I'm going to tidy up the instructions on my website, and then get on with something more rewarding.