After Andrew's comments about noise, I thought I would try converting the original RAW file in Fuji Hyper Utility, instead of Raw Therapee, to see whether the noise in the sky would be any less.
I tried to do much the same PP work on this WS, and although the sky is not quite as blue, I think the noise is far less. I haven't bothered to clone out the wires and aerial with this shot.
I have not been using RT for very long - I am using the "crisp" default setting - I think I must be doing something wrong to get this noise.
Any comments, especially from somebody who is using RT, would be very welcome.