i don't think there are now- there has been one every week since the first week of Jan- but now they're pushing the slim for life- i don't think there's free membership- just an offer if you buy a 6 week countdown (or 12 weeks)
i believe you get free membership if you buy 12 weeks in one go (which is what i did when i joined- never looked back) but other than that it'll be either half price membership or a free week- i've never seen anyone pay more than a tenner in the first week to join up x