Thanks for getting back so quickly David.
I've read further on your suggestions and thought a bit more about our needs.
Thinking about how many users will be logging in at the same time, I don't think it will be 1000's, probably not even 100's and we do not require any reporting on who has taken the course, how they did in quizes etc.
So the simplest option I can think of is to allow guest access. Would this cause the same sort of problems you described above?