More detail from the TfL Pension Fund Staff Travel pages:
1. Am I eligible for a Staff
http://www.tfl.gov.uk/microsites/pensions/documents/staff-travel-cards-faq.pdfTravel Card?
In order to be eligible for a Staff Travel Card you must meet all 3 criteria listed
below: (if retired you must hold a 1st class PTAC.)
You must be in service with TfL before 1 April 1996.
You must hold a Privilege Ticket Authority Card (PTAC).
You must be of an eligible grade (Band 2 & above).
(So as you need a Staff Travel Card to get the free Stena travel, effectively this means you do need a PTAC. Unlike most safeguarded TOC staff, the PTAC and Staff Travel Card for the free trips for qualifying TfL staff are separate documents for some reason, and you can't use the STC as a priv card on its own. The Staff Travel Cards FAQs
http://www.tfl.gov.uk/microsites/pensions/documents/staff-travel-cards-faq.pdf has more details.)