Like an idiot, it did not occur to me that I could just google the line, until after I had posted this query! This would seem to be the poem - misquoted by Lawrie. It is Walter de la Mare:


"Sweep thy faint strings, Musician,
With thy long lean hand;
Downward the starry tapers burn,
Sinks soft the waning sand;
The old hound whimpers couched in sleep,
The embers smoulder low;
Across the walls the shadows
Come, and go.

Sweep softly thy strings, Musician,
The minutes mount to hours;
Frost on the windless casement weaves
A labyrinth of flowers;
Ghosts linger in the darkening air,
Hearken at the open door;
Music hath called them, dreaming,
Home once more."