The method for changing to a new feed involves adding a special tag to the feed as described here:
http://www.wilmut.webspace.virginmedia.com/notes/podcast.html#move
Please make sure you fully read and understand the article before you proceed so that you don't risk painting yourself into a corner (do come back if there is anything at all you are not clear about).
There is a further complication because iWeb won't allow you direct access to the feed, so you will have to edit it outside iWeb.
Your signature says you have iWeb '08 so you must be publishing to a local folder and then FTPing to your server using a separate FTP program.
You will need to locate the feed. The URL you give is wrong: your feed is at
http://www.mod-life.net/Filmsploitation/Podcast/rss.xml
Open this feed in TextEdit in PLAIN TEXT MODE (not Rich Text which is often the default). Add the tag as shown in the page I linked to. Save with the same name to overwrite the original. (TextEdit will offer to add .txt to the filename - don't let it, it must be rss.xml.) FTP your site (or just the feed, making sure to replace the correct one).
Having done this do not upload anything else to the old site or you will over-write the feed. Leave it there for two weeks.
Before you do all this make very sure that the new feed works - subscribe to it manually in iTunes and check everything is in order.