Hi Sam,
I can think of one simple way of doing this, but it might not be what you want. Still, here it is.
1. Separate your questionnaire into various views. Let's say you have 4 views, with (roughly) the same number of questions.
2. You have buttons to "Go to the next view" when a user completes the fields in one view.
3. Then it's simple: view 2 says 25% complete, view 3 says 50% complete, view 4 days 75% complete.
I've heard of an Active X control to show a progress bar, but I've never implemented it and never seen it in action. I know it's linked from an old forum post.