Jani and the Great Pursuit by Eric Brown

Jani and her stalwart companions, Lieutenant Alfie Littlebody and Anand Doshi, find themselves chased from India, via Greece, to London by the British authorities, Russian spies and a Hindu priest - who all want what Jani carries, the ventha-di: the key that will open the door to other worlds. In London she attempts to rescue the imprisoned alien Mahran - the only person who might help her save the Earth from the invasion of the merciless Zhell, the self-styled Masters of the Cosmos.
But will she escape London and reach Tibet before the forces of evil capture her - and before she is betrayed by someone she considers loyal to her cause?
Jani and the Great Pursuit is the thrilling sequel to Jani and the Greater Game from best-selling author Eric Brown.