I don't need to issue a warning about editing the registry manually. Of course, this will change the default (boot-time) touchpad profiles for EVERY user, but since both my wife and I detest tapping with a vengeance, this doesn't really matter. For our model, it meant changing the value of 'Gestures' from 7 to 6, tho it may vary for other machines. To make the touchpad changes permanent, you need to change the defaults - AFAIK, this has to be done in regedit. I doubt the defaults were meant to be used this way (surely they should be more of a 'factory reset' function?), but that's how things happen on our Inspiron. However, each time the machine is booted, every user's individual settings are copied from the default, overwriting any recent changes they may have made.
By trial and error (changing the settings in Control Panel, then seeing what effect it had on the registry settings), I discovered that modifying a user's touchpad settings changes their INDIVIDUAL registry entries (so each user can have a different profile), but the default remains the same.
There's a batch of registry entries for each user's touchpad settings, as well as a single subset of these as a default (fortunately tapping on/off is in this subset). Sorted! In case anyone else hits the problem, here's how.