The Ex-Cav suit was a advanced piece of diving equipment used at the archaeological dig at Dunstanton Lake, where its operator was Johnny Repford.
It was taken over by Defence Advocate 997, who used it to steal the key to release the Dastrons from their suspended animation. The Dastrons later used it to go and connect the power cables for their spacecraft, but Mike Yates put a grenade in the suits' hydraulics section, destroying the suit. (AUDIO: Prisoners of the Lake)