Well... it took some searching!  It turns out that it's a poem by William Souter (1898-1943) set to music by James MacMillan.  I eventually found the text in an online article by Sheila Douglas,  20th Century Poets 1: Hugh MacDiarmid and William Souter; and here it is:

THE TRYST

(William Souter: 1898-1943)

Sae luely luely cam she in,
And luely she lay doun
I kent her be her caller lips
And her breists sae sma and roon.

Aa thru the nicht we spak nae word
Nor sindered bane frae bane
Aa thru the nicht I heard her hert
Gang soundin wi my ain.

It was boot the waukrife hour
When cocks begin tae craw
That she smool'd saftly thu the mirk
Afore the day wud daw

Sae luely luely cam she in
Sae luely was she gaen
An wi her aa my simmer days
Like they had never been.

Particularly poignant, as Souter suffered from spondylitis, and was bedridden from his early 30s.

Malcolm