He seemed to appreciate the performance although, like many in the audience, he must have hoped that there was more harmony back in the talks than there was in Messiaen's often discordant Turangalila symphony.
Many of those present craned to catch a glimpse of the Secretary of State for Northern Ireland who is still, in Belfast terms, very much a new boy. Although he has been in the thick of things in the background, his role has been largely overshadowed by that of the former United States senator George Mitchell.
He made an important contribution in the Commons when he pledged that any failure of the IRA to decommission would not go unpunished, a declaration that provided useful support to David Trimble in his campaign to convince the Ulster Unionist Party to accept the deal.
To the public he has seemed more of a cheerleader than a full participant. But the politicians know that he has been highly active behind the scenes, sending out some important messages to those involved.
It was conveyed to both Mr Trimble and Gerry Adams that they would be unwise to block progress in the Mitchell review in the hope of faring better in a later Mandelson initiative. There was also a more generalised Mandelson effect in the sense that his appointment sent the message, in the clearest possible terms, that Tony Blair intended to remain personally engaged in the peace process.
A recent photograph of Mr Mandelson with the Sinn Fein leaders Mr Adams and Martin McGuinness, both of whom were smiling broadly, is not to be taken as evidence of a jovial new relationship between the minister and the republicans. He happened to be on his way into the building as they were on the way out, and the smiles resulted from some passing pleasantries.
He appeared to be functioning at one remove during Mr Mitchell's review, watching closely and receiving constant briefings.
If Mr Mitchell had failed, Mr Mandelson would have been landed with the extraordinarily difficult task of working out what to do to save the peace process.
But the review was a success, which leaves him with the lesser but still delicate task of nursing the new arrangements along.
His job is now much reduced, responsibility for so much of the day- to-day administration having been transferred to the executive.
Mr Mandelson will, however, keep control of law and order and security, and will be expected to act as the new executive's protector and, in effect, guardian angel - helping to guide it through the many difficulties that still lie ahead.Reuse content