I have a new Phantom 3 but my iphone 6s will only display the app in landscape orientation and there is no way to clamp it sideways in the cradle as the sliding clamp doesn't move down far enough. Even if I clamp it in lengthways the  little fold down keepers on the side aren't far enough apart to fold down. Any clues?

There are some extension clamps, I know I have seen here, or on another forum. that you will need to buy. Check eBay for starters.
Unless that you might know someone with a 3-D printer to make you a couple. ;-)
Otherwise, a different device that fits. ;-)

RedHotPoker

I'm confused the iPhone 6s is the same dimension as iPhone 6 and that'll fit in landscape orientation in the holder with the clamps part way folded out. The picture here shows one with the sunshield but the principle is the same. Have you tried that?

What Mr Harris said

Thanks. I just worked it out myself so went straight to the forum to put it out there and you' expectations beat me to it. I thought the clamps were for holding the sides of an iPhone five vertically. Guess I'm a bit slow!

Thanks for the info