Andy, this is not the ideal kind of image for HDR. Your camera should cope with this without it, indeed you say that the base image was ok. HDR is best used where you've got extremes of highlights & shadows beyond the range that your camera can handle. Try it indoors, in the house will do to practice. Set the camera on aperture priority & the metering to evaluative (or multi-segment, or whatever Nikon call it) & do 3 exposures at -2, 0, +2. The rest is down to tonemapping & subsequent faffing aroud in PS. Not sure why your results are soft. Probably down to using a Nikon.
Hope this helps a bit.