Can the old style be accessed by a URL rather than having to have a cookie loaded by clicking Full Site.
AFAICS it can't, yet that seems to be exactly how the majority of other websites get around this issue. There are a few other forums I visit that have two URLs, one for the full version, one for the mobile version.
This is a fantastically easy to use and understand way of dealing with mobile users, and even the least technically competent person can usually see if they've landed accidentally on the "wrong" version, as the URL usually has the give away words "mobi" or "mobile" in it, as well as giving an option right at the very top of the mobile version to view the full version.
From what I can see, PPRuNe is using some novel way of trying to guess whether a user is trying to use a mobile device or a large screen device, and my experience is that, under some conditions, this is failing miserably.
At a guess, thousands of laptop users will want to access this site from wifi hotspots, tethered wifi devices or other wifi connections that may not go via their home landline connection, yet many will get an unusable version of the cut down mobile site, without the option to get back to the full site, if my experience is typical.