This discount is available to those on a £1 month's Which? trial, but not current members. [Thanks to Mottman for the correction.] (Link is here and on the checkout page.) Took me all of 3 mins to sign up, and hey, maybe it'll even be useful! The page is a bit sparse on specs, but it has an FHD 15.6-inch IPS screen as well. Please see the Argos page here for some Q&A with people from the Medion team.
I know, I know, no SSD, only DDR3, but hey, it's under £500! Looks to be retailing anywhere from £580 to £650 and upwards elsewhere. Used Laptops Direct in the past and had no problems.
Have a look here for a comprehensive test (if it translates for you). Hope you find this deal useful :)