I've been endlessly tweaking for the last two days. I'm making progress as I think the sun has improved, as has the texture. It feels more like a sandy landscape with the ripples than a rich soil which the previous render felt like. It's getting there, but a ways to go yet - some texture issues which need addressing such as the blend from the ripples to smooth which I've no idea how to do at the minute, etc.
In my quest for texturing, I've also lost the lighting that I was going after but will come back to that with Matt's suggestions once I've nailed things a bit better.
Any road, FYI, here is the update.