The Public Eye - Peter Shaffer

Charles and Belinda are an ill-assorted couple yet they were once in love. Insanely jealous, Charles engages a private eye, Julian, to follow her round London. Julian can only report that she is attached to someone. When the three meet it transpires that Belinda has fallen for Julian. Deciding to mend a marriage, rather than break it, Julian banishes Belinda to her wanderings but this time to be followed by Charles.