I am excited for you! Nothing like a litter of yorkie puppies! Do you have all of your supplies ready? If you need any help as to what to have on hand, just ask. I don't know anything about whelping bull terriers, so I am not sure if they are close in similarities or not. I would think the experience would be very beneficial.
I am close to you -- just sent you a message on your profile.
So, if you need anything, just holler. I have only been through whelping two yorkie litters. You probably know as much or more than I do since you have bred small dogs before. I did a lot of research, but nothing like experience to really keep the worry to a minimum. They ARE our babies and to worry is natural -- can't take it all away no matter what.
One of the most important things I learned that nature does not seem to take care of very well with our yorkie mommas is calcium supplements. Ask your vet about having it on hand. He can give you the prescription stuff to give orally. My vet actually prefilled syringes so it was very easy for me to give them in her mouth. In lieu of that he said she could have half a Tums each day. But you don't start this until AFTER whelping.
I did let my momma take care of tearing open the sacks and chewing the cord until she had a problem, then I tied it off in two places with thread and cut in between. Some experienced breeders will tell you not to leave it to momma as they have been known to chew part of the pup or otherwise injure them. Many of them will tie off the cords for every puppy. Find posts by Mardelin (Mary) -- she has the best detailed explanations of what she does to assist every birth.
Have you used the Veterinary Emergency Referral Center/Hospital on N. Davis Highway before? I have their address & phone on my emergency numbers list but have not been there for any of my pups yet. (Thankfully!) Good to have one on speed dial but also good when you haven't had to use them. I am going to go there one of these evenings just to check the place out. Looks really nice on the outside and they have vets from the area come staff it, even specialists on call. I mention it just to make sure you do have a 24 hour vet identified just in case. Hopefully, as I said, you won't need them, but good to know they are there.
Wish you and Bella Donna well!