They Came From SW19 — Nigel Williams

At fourteen years old, Simon Britten is a keen ufologist. When his dad dies, two things happen: a UFO-watch on Wimbledon Common turns up something distinctly out of this world; and his mother joins a local 'church' and attempts to contact her deceased husband beyond the grave. Caught between the Extra-Terrestrial and the First Church of Christ the Spiritualist, Simon struggles to work out just what - or whom - he should believe in.