Distribution system operators are starting to implement market-based mechanisms to use flexibility from distributed energy resources. Flexibility tenders allow distribution system operators to procure flexibility for the medium to long-term, with the first implementations in Europe in the UK since 2018, and a recent pilot in France. This work seeks to quantify the potential participation of electric vehicle aggregators in a flexibility tender, considering the role of market rules and product definitions. The main parameters that affect fleet participation and remuneration are the bidirectional capability (V2G), the reliability of the fleet and the match of availability profiles to the tender requirements.