Spoke to soon ... so now I've added the horizontal connects, it's really obvious which sections of geometry are subdivided and which arent. See new attachment. The problem being, adding the horizontal connects has just shifted the problem to the larger flat planes that now have tonnes of horizontal connects.
The only way I can see of getting the displacement to be consistent all over the model is to have perfectly sized even quads everywhere, which simply, isn't practical. Basically the more subdivision, the better the displacement (or more pronounced/accurate). But it seems like such a waste of polycount?
So i guess the question is what's the best approach to get consisten displacement across an entire model, making the mesh ultra dense seems silly where it's not needed but if thats the case then the density is going to be controlled by the curved areas and the number of segments needed to keep them looking curved.
I also can't tell which displacement is meant to be correct now either. Is it the subdivided section or the flat non-divided areas - as they clearly look different in terms of displacement depth/strength.
Finally this new issue shown in the screenshots is not half as obvious with the old displacement - as you can see in the attachment, the old displacement looks much better and the issue is nowhere near as obvious.