def replace_nhs_numbers(fn, locations):

    outFn = os.path.join(os.path.dirname(fn), "ANON_" + os.path.basename(fn))
    with open(fn, 'rb') as f, open(outFn, 'wb') as out:

        for location in locations:
            buf = f.read(location - f.tell())
            out.write(buf)

            pseudo = str(get_pseudonym((int(f.read(10)))))
            out.write(pseudo)

        out.write(f.read())