Problem solved! Finally managed to talk to the correct group at Comcast today. It was a five minute fix on thier end to one of thier network routers. We can now get through to our nameservers, and all sites are resolving. Everything works.
Thanks for all your help Falko!
I hadn't though of a technical domain, but I will keep that in the back of my mind should the need come up in the future. This was frustrating tracing all the various things down, but a good learning experience. I've got one windows machine left...... May get rid of it now. Linux I think is actually easier to deal with!