I subdived the decking first and then smoothed the edges (with soften coplanar ticked) in order to weld them for use with displacement - otherwise there will be gaps where he main faces join.

Ordinarily I do tend to bevel edges on geometry that I intend to displace (or any sharp edges close to the camera). In this case however I wanted to be as economical as possible and see how far you can get with very simple geometry.

In other words the lesson learnt is, always bevel the edges of rectilinear geometry you wish to displace......there are no shortcuts!

As Notareal has pointed out over at Sketchucation this problem is actually down to normal smoothing being checked in the displacement settings.

The weird thing is I tried unchecking normal smoothing when I first spotted the problem and it made no difference. Today however it works just fine

The decking still looks better with the bevel and I should know better than to cut corners.