def find_nhs_numbers(fn):
    try:
        f = open(fn, 'rb')
    except IOError:
        return None
    locations = []
    num = 0
    while True:
        c = f.read(1)
        if c == '':
            break
        ascii_ = ord(c)

        if ascii_ in (48, 49, 50, 51, 52, 53, 54, 55, 56, 57):
            num += 1
        else:
            if num == 10:
                startLocation = f.tell() - 11
                f.seek(startLocation)
                if validate_nhs_number(f.read(10)):
                    locations.append(startLocation)
            num = 0
    return locations