Thanks for all the replies. I've found
www.ISPConfig.org and I'm going to have a look at that to see if it will help manage the different accounts.
The basic idea is to be able to offer many other people web sites so they can ftp their content into their home directories but not be able to log on or do anything else.
Obviously they may want to change their password from time to time. (yes, password reset is a different problem - but an important one too).
I haven't looked at ISPConfig yet, but it looks like it may help.
Thanks.