Hi Nicko,
You will find all the details at
Alsa/Enatcar site.
There is only 1 "Clase Supra" bus a day between Madrid and Benidorm (search "Supra" at the transportation section and you will find comments on this kind of buses).
Regards,
Antonio
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The best tips from your favourite hostal in Madrid.
Hostal Chelo at
http://www.chelo.com