You shuld be able to see an 'edit' button between the 'report' and 'quote' buttons in the bottom right hand corner of your post.
Thanks dabloodymess, but it looks like my edit button is missing then. There's nothing between report and quote on my screen.
There is a time or post limit before you can edit the thread as you are new, you'll have to post, 10 posts before you are able to edit them.