A bit late but:
You can fit a longer seat post to the Cnoc 16 (original is 200mm long). Your Beinn has, I believe, a 250mm long post.
My daughter has a 490 inside leg, so should be ok (on her Cnoc 16) according to the specs; but has her seat set high & has run out of post.
If not then Islabikes will sell, with some prompting, a 250mm post for a very reasonable £8 plus a not so reasonable £5 for postage. But it did arrive less than 24 hours later.
If the reach is too long on the Large Beinn: how about turning the seat post round & then pushing the saddle as far back as possible, tempoarily? I am not sure what this would do to the peddle / seat alignment & whether the head angle would allow. But looks as though it might be ok on the pictures.