obviously living in a rural area. apparently if you where living in a town or city, you would expect a phone mast approximately every 200m for each phone company.
if you wanted to object to the mast on heritage grounds, the mast would have to impinge on the views of the castle. (the radio waves obviously won't damage the castle in any other way.)
check the development plan for the area. it may have protected views around the castle on which to base you objection.
Do remember however, there is a mobile phone for every man woman and child in this country. without the masts they won't work. and they do have to be in somebodies back yard, and 600m is quite a distance away from your house.