Pete
Charlotte is spot on with her advice. Viewing certificates at the local office generally costs the same as ordering a copy (7 pounds), and at least with a copy you get to keep it.
For Kent records generally, head to the Kent Family History web site (
www.kfhs.org.uk. They sell CDs and microfiche covering most of the county, and very reasonably prices, considering the amount of information they contain.
There are a few pockets of online data around for Kent, but not many. Generally private websites with very limited scope.
Your other option is your local LDS Church Family History Centre. They also have many microfiche (and access to more). There are more in that format than online at the IGI site.