Very strange.... Maybe I should have mentioned this, but I was using IPR, but just to see what happened I switched to normal production render and suddenly it renders the gravel perfectly.. Now that's weird. As I've said before, I've never seen it render pink gravel before and I have got into the habit of using IPR and progressive rendering, to render every scene I have created. As for memory, this is one of the smaller scenes, so it's definitely not that.. and is only consuming just over 8Gb (I have 32Gb).. Many of my latest scenes, consume over 12Gb and render just fine using IPR.
Something for me to to be aware of in future I guess, but the thing is, rendering using IPR, if I spot something odd, that needs a small tweak, colour, light etc., then I can simply adjust it and let it run. But hey, I guess IPR isn't really the way to do production rendering and is really only for tweaking a scene, getting materials and lighting looking right, then you are supposed to run the production render.
Still confused as to why it's never done this before though, possibly something to do with a recent update, who knows, but all is well now