I may have to take the long road on this one then and go back to real basics, and stop trying to keep lifting heavyish on DL's for now.
With conventional I was lifting very much with my back, which was causing rounding at heavier weight where I was starting with my hips quite high, and my shoulders quite forward of the bar to make sure my back was flat, but my core just couldnt keep up and bingo rounding.
I want to bring my hips lower, to shift the weight off my back, move my shoulders back over the bar, and use more of my legs/PC, but when I do move lower I get rounding, regardless of how braced my core is, due to what I have been advised as quite shocking hip mobility.