That's one good-looking cross-frame bike! And in the very best colour too, British Racing Green, elegant by day and by night. Love your tan sidewalls!
About your chain tension: The one foot on the ground, other foot on the pedal, test already advised is the good one, which works on my bike with just over 10,000km on the clock.
However, I must agree with everyone else, your chain is too tight. Rohloff advises a very slack chain, and there is lots of reported experience from people running visibly much slacker chains than they would on their non-Rohloff bikes.
I don't know what adjustment is available on the Singleator, or whether turning it around as Martin suggests would add enough/any slack, but if that fails or is impossible because of the geometry of the bike, I would suggest you start at the Singleator's lowest tension and ride-and-adjust until the chain doesn't fall off.
Alternatively, if it is easier, just add a couple of links to the chain, ride-and-add-links until the Singleator permits the chain some visible slack.
A Rohloff-equipped bike which depends on an eccentric bottom bracket or Rolloff-designed axle hangers in slider slots should have a chain with visible slack, a drop of say 10mm vertically in the middle of each run of the chain.
Though the Rohloff change gets easier as it beds in, which happens over an extended period (a Rohloff is run in about the time a Shimano Nexus box lies itself down to die), eventually it becomes second nature to the rider to lift slightly when changing gears under load. At about 6–8000km I perceived a difference between changing gears at speed on a flat road which at that point didn't require liftoff, and changing gears on an ascending road, which still requires a slight lift.
I wrote here once that if I were to sell my Rohloff HGB, I would demand a premium over the new price for the service of running it in. If you have experience of the Shimano Nexus, in which the gear change isn't very precise but the control, properly set up, is not as loose as even a new Rohloff, forget it: the Rohloff will never be as smooth a changer as the Nexus is capable of (I know because I also have an electronically automatic Nexus box, the full Di2, not the cut-down assisted manual of the Dura-Ace groups, and my Smover changes as smoothly as Shimano promised, almost imperceptibly), but the Rohloff will get near enough, and the precision it starts with is about the same throughout it's life because it is just a better-made box.
Analogously, from the beautiful fitting of your noodles, I wonder if your gear change cables aren't a bit tight, as in road bike derailleur practice. Rohloff cables are supposed to be set so that there is a minimum of 1mm play around the mark for the gear the box is in. My box, still on its first cable set, has always had 3mm of play to each side -- and it was set up like that by an experienced and conscientious (German) mechanic with factory training.
In the beginning, when I marked the crucial gear indicators 1, 8, 11 and 14 with white Tippex, I wondered at these tolerances on the cables and the chain but it soon turned out that they're fit and forget, and I think of them, now that I'm changing gears without thinking about it, only when I run into a new Rohloff owner with the same concerns. The only gear that I can ever tell you I'm in is the 1:1 gear 11 which is what my respiration rate and heart exercises are predicated upon, which I select by going to gear 14 and clicking back three slight notches. Otherwise I'm just magically in the right gear for the lay of the land and the rise of the road.
A happy Rohloff owner is one whose rotary gear change control numbers are worn almost to invisibility -- or beyond.
