sweep thy sad strings?

[antfan.livejournal.com]

Wading through the chapter in the Marlows and their Maker that lists all the literary references, reminded me of the one I've always wanted to know the source of - and typically, its not listed.  So I'm hoping one of you literary types might know.

It's Peters Room, p55 of my Faber paperback, and they are roasting chestnuts in the Shippen and Ginty says it has a   
"sort of Great Hall feeling - the fire in front and everywhere else madly dark and cold."
"Like the sweep thy sad strings musican one," said Lawrie unexpectedly.  
Ginty agrees and says "we've even got the old hound whimpering in the corner"

Anyone any idea what this - poem, presumably - might be?