I have always thought that the back road which runs alongside the Royal Academy, behind the Society of Antiquaries, is one of the more mysterious spaces of central London, a phantom space unused for most of the year, like Down Street, the forgotten stop of the Piccadilly Line.   But the road is opened once a year to allow for the delivery of sculpture to the Summer Exhibition, so tonight there was a view down what may be the longest unadorned wall in Europe to the gate which was open onto Piccadilly:-