I took some advice from Col & Zig and my ride improved quite a lot.
Here's the video by them:
I have, in more recent times, cranked up the rear pre-load a few more notches and got an even better result.
So.......If your rear suspension is jarring your back, over bumps in the road, my advice is keep cranking up the pre-load a bit, until you reach an acceptable ride to the 'bump-stop' and then adjust the damping accordingly.
This bike is more sensitive on the rear end than any other bike i've ever owned, but at least there is plenty of adjustment available, to get it dialled in to suit my body weight.