Well I did reach Dennis at Athenstours through the email address (email@example.com) by Almostretired (thanks again) and I have made a confirmed reservation for two transfers, one to the hotel on the morning of arrival in Piraeus and another to the airport from the hotel on the next day. Somehow I believe that the link one finds at http://www.athens-limo.com
doesn't work as I never recieved a single response from my two attempts at communicating with Dennis through that link. The fees for both transfers are very similar to that offered by the hotel. I wonder if the hotel also would have booked the transfer through Dennis' firm. I'll ask when there.
Dennis also let me know some telephone numbers and his email address in the event of a taxi strike in Athens or difficulties at the port so that alternate arrangements for the transfer could be made. Very reassuring.
Thanks to all for some sound advice.