Bannermen

The Bannermen were a gang of terrorists led by Gavrok.

Tactics and Technologies
The Bannermen, led by Gavrok, were a military group that took no prisoners. The Bannermen Warfleet was composed of large three engined cruisers. Their technology included sonic cones, bazookas, fragmentation grenades, special restraints, able to hold two at once, magnascopes, ray guns, and sniper guns. Their weapons were mainly focused on atomising enemies. The main gun used by the Bannermen has explosive bullets. (DW: Delta and the Bannermen)

History
After polluting their own world, the Bannermen attacked Chumeria in an attempt to kill all Chimerons and take their world. They killed all the Chimerons, except for Delta, the Chimeron Queen, who escaped and ended up on a Nostalgia Tours bus along with Melanie Bush. Gavrok and a squad of Bannermen pursued her through time and space to the Shangri-La Camp on Earth in the 1950s.

Gavrok, and the squad of Bannermen arrived at the Shangri-La Camp to hunt her down and kill her and her child. They were defeated by the Doctor, Mel, Ray, Billy and Goronwy. (DW: Delta and the Bannermen)
 * It is unclear if the Bannermen were a species or just a mercenary group, since their leader actually led them to kill Delta. No reference is made that Gavrok answers to a higher power.