In my very humble opinion a big old smelly dog fox in December/January with his blood up and fanny on his mind is a whole different ball game to the daft saplings in September, I owned a beddy/whip x grey(23",19kg max) who took to em with fire and brimstone.....he was bought in as a ferreting dog, but showed he had the minerals, although he'd never finish the job as quick as a purpose bred dog, but handy when needed.
Cheers, matt