OK - thanks for that!
It may be that I can use an embedded form on our own site rather than the landing page. My requirements are:
1) pre-populate hidden fields with default values set in the form
2) populate one hidden custom field using a token
For #2, when the user clicks through a campaign email, this should set the token value so I don't think I need to use the landing page? Or do I?
Still a bit of a newbie here - thanks for bearing with me.