I agree with fsmnutter. The sheep are on your land and the authorities legally need a clear line of communication with the flock owner otherwise you could be involved if there were issues with tag numbers, welfare, outbreaks of disease etc etc Whether it is his original flock number or a new one isreally not important (except perhaps psychologically to him) but what does matter is Defra know who he is, and his flock number is associated with the CPH number relevant to your fields.