Hi Graham
I use both Wills and Slaters sheets for scratchbuilding, and with some of the Wills sheets the joins are easier to hide than others (Wills roof tile sheets are a nightmare to disguise when joined).
I use a lot of the Wills coarse stone sheets and what I tend to do when they're initially joined is paint the area round the join in a light grey which then shows up bad joints and then put in plastic putty or Milliput around the join, and when dry spend a lot of time carving fresh joints between. I then put on a little more light grey paint again to see if there are visible joins still, if so then a little less filler, carve again when dry and paint grey again and inspect.
With the photos below, you can see how I used Wills coarse stone on a model of a good shed. As mentioned above, I made the join where there was least area. You can still see evidence of a join on the edge.
On the pictures below, you can see before and after shots of a tunnel mouth on my layout. A bit of filler and careful painting disguised things (I hope!)
Finally, the shot below used Wills random stone for the wall. Again, I made the walls, grey paint, filler, carve again etc and when satisfied paint properly. - spot the joins.
Hope this helps
Best regards, Edmund