Crackin reply Wink Hound.
I've just looked into where I first read about greyhound foxes and it came from Plummers Fell Terrier, now for all his ramblings, it's the one book of his worth merit, if only for going to the lenths to make it.
What it states in there is that, though many old time fell men belived they did exist, no evidence has ever been produced, he also states that foxes running over a fell side do look much bigger due to terrain and that diet, being more sparce for the fell fox than southern foxes, would add to this 'illusion'.
All food for thought....!