I can really recommend Paul Proctor from Simply Horses based near Houghton-le Spring. He was part of the Young, Proctor and Wainwright equine practice for years (based near Hexham - now called Bearl Equine) but has now set up his own practice. The call out fee is much lower as he is so close and you are pretty much guarenteed to get Paul himself!
I really reccomend Durham Equine Practice, call out reasonable in comparison to other vets and always been able to ring up and ask for advise etc as well.