Hi there - Hehe, thanks for your comments

Yes, I prefer bunched and thicker pops too, but my machine doesn't

I was rendering with 3 high-qual (not highest-qual!) pops - no more than 4,500 instances between them all
Qual 0.9, AA 5, GI 3/3
SubDiv Cache 200
Min/Max threads 1/16 (default)
1 cumulus layer (fog) 46 samples, atmo 64 samples
rendersize as you see it (1280x1024 more or less!)
The blotch didn't occur in any of my smaller previews - only in the right-hand cropping.