Kosten played many tournaments, finishing first or equal first in the following: Perhaps the most striking of these performances was at the 1st Geneva Open tournament of 1986 (170 players, including several grandmasters, among them Anthony Miles, Miguel Quinteros or Armenian Smbat Lputian from the Soviet Union).

In the final game, Kosten accepted a draw offer from his compatriot, Glenn Flear.

IM Kosten won outright a full point ahead of second placed GM Lars Karlsson.

With the English national team he participated in the European Championship in 1989 in Haifa, where he received the individual bronze medal for his score of 5 points from 7 games on the first reserve board.

In Switzerland, he played for Lausanne Le Joueur, and he was also active in the Hungarian (for MTK) and Basque (for Oaso X.T.)