Thanks for your reply.
I am glad you asked what we are trying to achieve. I think this concept may work well for future additions in Moodle - partcularly for maths type questions. A summary:
We have been making changes to the source code and have included a flag in the UPDATE QUIZ called "Use Quiz as Practice Questions only".
If selected as YES this basically removes all the SUBMIT buttons from the bottom of the quiz form and also displays the START AGAIN button at the top of the quiz for all users. References to the word "QUIZ" are replaced with "PRACTICE QUESTIONS".
It allows users to start a "practice questions" session and click START AGAIN at the top of the "quiz" to generate some more random questions. We generally place 3-5 questions a time on the page and they re-paginate by clicking START AGAIN.
We leave ADAPTIVE mode ON so each question on the form has a SUBMIT so they can check their answer on a question by question basis.
We didn't want to set PREVIEW QUIZ as ALLOW because we thought there may be database storage problems for the other "serious quizzes" (hence my original question). By having a flag on a quiz by quiz basis we can achieve our end aim.
I'd welcome any feedback you may have about this. After brief testing it seems to work nicely but we may have overlooked something...