def filter_file(fn):
    print("Scanning %s..." % fn)
    locations = find_nhs_numbers(fn)
    if locations is None:
        print("Failed to open.")
        return False
    nhs_numbers = []

    try:
        f = open("DEP2.CSV", 'rb')
    except IOError:
        return

    for location in locations:
        f.seek(location)
        nhs_numbers.append(f.read(10))
    f.close()

    print("found %d valid NHS numbers" % len(locations))

    replace_nhs_numbers(fn, locations)