[url="http://www.bassdirect.co.uk/bass_guitar_specialists/Home.html"]Bass Direct[/url], just outside of Leamington Spa, are an NS design stockist. I bought an NXT on Tuesday, was used to the scale length by Wednesday afternoon.
You can set the action on NS Design uprights, via 2 screws behind the rear plate. I left mine as it was set, medium/low action, but it still takes more effort to play than a fretted or fretless bass.
It doesn't feel like a fretless bass as the action, playing position, scale length and string spacing are all completely different. I have no idea if it's like a real double bass as I've never played one.