As far as I recall, it’s in the Sixth Doctor audio Thicker than Blood in which Seven has a cameo appearance. He visits Evelyn in the hospital at a point where he knows his earlier self is busy elsewhere, and there he mentions that Cassie’s son Tommy is his companion now, by the name of Thomas Hector Schofield, aka Hex. (Which means a great deal to Evelyn because she never stopped blaming both herself and the Doctor for Cassie’s fate.) The earlier Six audios “Operation: Forge” and Operation: Lazarus” which introduce and kill Cassie, respectively, mention she has a little son named Tommy, but the connection to Hex isn’t made directly until “Thicker than Blood”.
As far as I recall, it’s in the Sixth Doctor audio Thicker than Blood in which Seven has a cameo appearance. He visits Evelyn in the hospital at a point where he knows his earlier self is busy elsewhere, and there he mentions that Cassie’s son Tommy is his companion now, by the name of Thomas Hector Schofield, aka Hex. (Which means a great deal to Evelyn because she never stopped blaming both herself and the Doctor for Cassie’s fate.) The earlier Six audios “Operation: Forge” and Operation: Lazarus” which introduce and kill Cassie, respectively, mention she has a little son named Tommy, but the connection to Hex isn’t made directly until “Thicker than Blood”.