As MacMillan strode on to the stage, the place erupted with a-whoopin' and a-hollerin' that is not the accustomed experience of contemporary composers, even, I suggested, our most successful modern one.
The fact that the largest contingent of youngsters in the hall were from MacMillan's alma mater, Cumnock Academy, was one reason for the cheering. Perhaps so, but last weekend I found myself in another venue in the company of the same composer and something very similar happened, without any such obvious explanation.
The concert at Greyfriars Kirk in Edinburgh was the last date of The Choral Pilgrimage 2013 by chamber choir The Sixteen, which began in Guildford Cathedral back in March. The programme, entitled The Queen Of Heaven, had only one other Scottish date - Perth Festival of the Arts in May, prior to which The Herald carried the fruits of a conversation I had had with the group's founder and conductor Harry Christophers.
The Queen Of Heaven programme paired the music of Giovanni Pierluigi da Palestrina with that of MacMillan, adding Christophers's new revision of Gregorio Allegri's Miserere, arguably the most popular single piece of early choral music in recent times, to that rich mixture. MacMillan, Christophers told me at Greyfriars on Saturday, is the "saviour of church music" now, just as Palestrina had been during the Renaissance. The 2013 concert eloquently and unequivocally argued that case, with the Scots composer's own version of Psalm 51's Miserere text and a selection of his beautiful motets becoming every bit as essential to the glorious success of the programme as the celebrated early music it was woven between.
It was standing room only at Greyfriars and the merchandise stall was a rammy at the interval as people stocked up on The Sixteen's chart-topping discs, so MacMillan's presence towards the front of the audience had very likely passed unnoticed by most. When conductor Eamonn Dougan invited him up to take a bow, the reception may not have been quite as shrill as the one in Glasgow's big hall in front of the Scottish Chamber Orchestra, but it was no less startling - cheers and stamping as well as applause, as folk announced their glee at discovering the man was in their midst.
In less than a year's time, MacMillan will be presiding over a new festival he is creating in Cumnock with his wife Lynne. He has been clear they intend it to become what Sir Peter Maxwell Davies made St Magnus in Orkney or Benjamin Britten built in Aldeburgh. At the start of last month it was launched at a hotel in the town where the young composer heard his first chamber music recitals. There was music from school children, some of them very possibly the ones who had hailed him in Glasgow a couple of weeks previously, and another local musician, violinist Nicola Benedetti from West Kilbride.
That day MacMillan told me that, if The Cumnock Tryst proves successful, he intends to devote the rest of his life to the project. Support for it has already come from all quarters, and the large number of people in senior positions in the arts who made their way to deepest Ayrshire for the launch event, which kept many of its cards close to its chest, spoke of the composer's acknowledged status - but the affection and regard in which MacMillan is held by the people of Scotland will be as crucial as anything to its success. The programme is under wraps until the spring of 2014, and it will be no surprise at all if the opening choral concert features the choir who performed his music on their Pilgrimage this year.