Our vet is in Dereham (http://www.larwoodandkennedy.co.uk/ ) and is 24 x 7 although you pay call out fee, not had any issues over the years but we have dogs. We did have to get our dog referred to specialist care for her epilepsy recently.
There's a big vets up near lidl/busseys and they have various specialists there as well. The one in Dereham closest to the market place is good as well