Thank you Nicolai I used your method to trouble-shoot the problem, having identified the number of actual vertices to be welded to a single point to confirm whether there were multiple faces.
The polies complied to weld when target welded with a box in the same element but did not solve the issue, having turned displacement mapping off with only bump and diffuse map materials switched on, a seamless edge would appear. Conversely the noticeable gap appeared when displacement was switched on again.
To further rule out the mesh topology issue I rebuilt the faces retrofitting the new ones in place with a single poly then applied the material with displacement map toggled and the same noticeable gap appeared in render.
I am now unsure whether displacement maps can span multiple faces in a single poly at different angles from the camera without noticeable gap.