Beyond the Galactic Lens. Cap Kennedy Book 16

Three times had the scientific genius, Kaifeng, slipped through the hands of the men of FATE - and three times those equally fanatic guardians of the fragile structure of interworld peace had tracked him down again. But now Kaifeng had something that the Free Acting Terran Envoys had never met before. He had a ship beyond all previous capacities, he had a crew of dedicated devils, and he had FATE's finest operative as his hostage.
And when FATE pursued him beyond the very Milky Way itself, beyond the Galactic Lens, things changed very rapidly - for out there Kaifeng had the means to enforce a stop to human progress - and he would not hesitate to use it!It's a super-space-thriller in the Star Trek tradition!