Hi Colin,
Examining the nose of a large dog can be daunting for both the vet and the dog!! Some dogs react badly to having their noses, etc poked at, and become aggressive. this does not always mean they are a dangerous dog. but, your vet is within their rights to refuse to see you. you could either offer to muzzle the dog, or even sedate it before going back, but if they still refuse, then you would need to contact another practice, tell them the history, and again offer to muzzle the dog before going in. You need to sort this out, for the dogs health-it will need vaccines, and hopefully never, may need the odd bit of surgery for cuts, teeth, etc. as for the problem, sounds a bit like canine acne, or a staphylococcal pyoderma to be more precise. This does need treating, as can persist and even spread. Please try again with your vet. Good luck.