Nick,
your on your phone but logged on to main site I presume?
Go to home (topleft)
Then scroll down keeping to left hand side where you'll see moderators etc below that there's a section, hard to describe but it's where you can change how your page looks when you sign in, you want the bottom one that says SMF 4 mobile or similar then underneath that option it has a tick box for permanently, obviously tick that then press apply. voila you should arrive back in mobile land.
Oh BTW, it's irrelevant if your on full site or mobile site regarding your data, that's a WiFi issue
And he reason you keep going to the full site is as when it's on mobile site you hit the button at the bottom of the screen that says "go to full site"
(Don't ask how I know that!)
Good luck fella.