Sheila Gold, supposedly Britain’s most accurate psychic, wants to be taken seriously by her new clients. “This is not theatre,” she warns them, as she lights seven candles for a seance. This is an insider joke. Theatre is exactly what it is. It is the kind of theatre in which the lights dim, then dim some more, a bell tinkles of its own accord and drowns out the ticking of a clock, while the audience sinks into a special kind of silence, still and focused. If it were not theatre, there would be no illusion. The Psychic marks the return of Jeremy Dyson and Andy Nyman after the spooky stage and screen success of Ghost Stories . Now the writer-directors are unnerving audiences again with grinding sound effects and sudden lighting bursts, toying with us to believe and yet not believe in voices from beyond the grave. Dark and light … Eileen Walsh as Sheila Gold in The Psychic.…