William Carr 
The Gypsy Quest [EPUB ebook] 

Two young people meet in a Welsh village school and become firm friends. Mirela is of Romany descent and possesses the gift of insight. Chris has been sent to stay with relatives while his parents work on an important project for the British government.
Chris becomes the target of a ruthless group of villains who intend to gain certain information which Chris unknowingly possesses. The villains attempt a kidnap, but Mirela uses her intuitive ability to foil the plot and the children narrowly escape. 
Eventually, after several more attempts to abduct them, they escape to Jersey, but they are captured and taken through war tunnels to a gun emplacement where the villains intend to force Chris to part with the information they want. A rescue bid is organised, and the emplacement is stormed. A gun battle ensues and while Chris and Mirela are being released, the disturbance provokes the spectre of a German soldier to appear. The spectre attacks both groups with murderous intent, until Mirela, with assistance, applies her psychic powers in an attempt to control the entity.