I have hot & cold water pipes run between the outside wall of an extension and the framed wall. I notice that the water is extremely cold coming from the bathroom tap. I'm worried about freezing. How can I remedy the situation without busting the walls open and insulating?

The water is cold because the weather is cold. When you run the water in the bath or kitchen it will be the same temperature. The only proper way to insulate the pipes on an exterior wall (and all of these pipes should be insulated) is to remove a small section of the wall. You could attempt to inject foam or insulation, but 100% coverage can never be guaranteed.