Know Before You Go
To find it, enter the children's hospital, opposite number 47 Great Ormond Street. At the reception desk, ask for directions to the Lagoon restaurant. At one side of the restaurant, you will find the instruments at a very small deck. They are only accessible through large sliding doors which are open when the weather is warm. In winter you can only look out through the glass, but you will not be able to hear anything.
Update July 2017: The pipes and horns are there, but there's been no music for years. Cafeteria/restaurant workers had no idea where or what it was. The deck below is now fitted with patio seating and is protected with netting which obscures the view. A knowledgeable employee said that children are told to whisper their wishes and prayers into the base of the tubes in hopes that they will fly to the heavens. Occasionally the music is turned on for "special groups of visiting dignitaries."