By the entrance to each room an elderly lady would be sitting on a small chair, awaiting the chance to flaunt her knowledge of the ceiling. There might be a small gift shop selling postcards, bookmarks and key rings.

If, however, one wanted to visit the house, one would run into a major problem: finding it. For it does not exist, and never has done.

No doubt a tour of the house would make a fascinating day out. One can imagine school parties going there and children filling in questionnaires asking things like:

You see, the rich Mr Flengate described in Mr Flengate’s diary is not Mr Flengate himself, but a fictitious alter ego. The real Mr Flengate was an indigent vagrant who eked out a meagre living as a circus acrobat and was often reduced to begging. But he was not illiterate, and always made sure he had a supply of paper on which to write every night.

After his death, his diaries were read, and it was discovered that they did not describe his life at all. It was eventually realised that they described its antithesis. Every good day for the false Mr Flengate was a bad day for the true one, and vice versa. Every positive thought the real Mr Flengate had was counterbalanced by a negative one for his imaginary double, and vice versa. Once this was understood, the diaries were seen to be almost unbearably sad, for the fictional Mr Flengate never encountered failure. Even his thoroughbreds won every race.