Hi & thanks
The answer seems to be that on almost all ADSL "modems" it is necessary to use "half-bridged" mode when going from PPPoE to PPPoA - so that the "modem" does the authentication stuff and leave the router in DHCP mode. From what I've found out this "half-bridged" mode seems to be a nasty kludge.
This is (apparently) the only ADSL "modem" that will do the job properly:
http://www.draytek.com/user/PdInfoDetail.php?Id=71M.