Yes, Ryanair seemingly has such an agreement. But I suppose they want the added protection that a greedy owner of STN might try it on and increase charges in future, as BAA did in 2006. They conveniently forget, however, that the 2006 increase in charges (actually an expiry of previous time-limited discounts) was sanctioned by the CAA - in that the regulatory price caps have always been higher than the prices actually charged by the airport. Stansted has never been able to price up to the maximum allowed by the CAA, unlike LHR and LGW.