The easiest & most simplest thing to do I found was…
1. Create your thank you page in the CMS, take note of it’s URL Key
2. Open up the following file in a text editor - /app/design/frontend/base/default/template/contacts/form.phtml
scroll to the very bottom & add the following bit of code:
elem = $("contactForm");
Save, clear browser cache open contact form on web site & test. Obviously you’d replace my thankyou with the URL Key created earlier on.
All other solutions on how to redirect the contact form just didn’t work for me.