I've added the validation to the thank you page, and it does appear to be redirecting back to my form page if it fails the validation. I also took your advice and used the Cookies Toolkit to re-populate the form (ta!).
How do I show validation errors though? When I'm returned to the form, upon fail, the user has no means of knowing which field is incorrect or, indeed, what the problem is.
Is this where the ASP/PHP relationship will cause problems?