Thanks Izzie. When I was cropping the original, I removed some vignetting from the corners (it was taken with a 200mm lens plus a 2x converter). I was tempted to crop some more off the bottom as you suggested because I thought that it would improve the composition, but I left it as it was because I wanted to include the dry stone wall in the foreground.
I have processed the image again from scratch, cropping more closely. When converting to monochrome, I used Silver Efex Pro's number 16 preset: Full Dynamic Smooth. How is it now?
John