Would the RSPCA (or more accurately SSPCA up here) be interested? There isn't actually an animal in distress....the feral beast seems quite happy to be terrorising the local cat population.
On another note, do you think you could take a cat in a fight? If it was HUGE and was really going for you? I'm not so sure. I think if I trap this beast I'll be putting on heavy duty gloves, goggles etc before I try and coax it into a box.
Since you're in Scotland, could it have been a European Wildcat? http://en.wikipedia.org/wiki/European_Wildcat