To all who replied so helpfully, thanks. I now have a working setup. Basically, the simplest solution for me is to add the "idle" option to my ppp options, to hangup the line when there is no more activity.
A number of people suggested diald - it certainly looks good, and I may well look into it again at a later date, but for now it seems like overkill for my (very simple) needs. Thanks once again for all the help. I'm really impressed with Debian (both the system and the community) - everything seems so well thought out and effective. Paul.
