Hello, I think I may have found the version you're looking for (though not having heard the Madrigal Singers, I can't be sure) on the
Skepticfiles page.

MEN OF HARLECH

Translated from the Welsh by:
Verses 1, 2: Peter John Stephens
Verses 3, 4: Thomas Oliphaunt
Verses 5, 6: William Duthie


See the glares of fire like hell there,
Tongues of flame that writhe and swell there.
Brave men strike with full-voiced yell there:
Forward with all might.
Armor clashing, cries of foemen,
Hear the chieftains urgin' "On men!"
Thunder of the charging horsemen
Echo height on height.
Arfon sings for ever
Of her might and glory.
Wales will be as Wales has been,
So great in freedom's story!
Those fires light up the sacrifices;
Cry of a dying Welshman rises.
In the cause of freedom's crisis
Bravest men must fight.

We'll not die, be conquered never.
Harlech, Harlech lives for ever
Freedom's from the Greatest Giver,
Freedom is our good.
See how Welshmen shouting run down
From the mountains they do come down
Like a storm that strikes at sundown
Boil up like a flood.
Welshmen's strength has made her
Freedom's strong crusader.
Swords of Welshmen have cut deep
The hearts of the invader.
The sword is met by sword replying,
Steel by steel on strength relying;
See where Gwalia's flag is flying,
Freedom's in her blood!

Hark! I hear the foe advancing
Barbed steeds are proudly prancing;
Helmets, in the sunbeams glancing,
Glitter through the trees.
Men of Harlech, lie ye dreaming?
See ye not their falchions gleaming,
While their pennons gaily streaming
Flutter in the breeze?
From the rocks rebounding,
Let the war-cry sounding
Summon all at Cambria's call,
The haughty foe surrounding.
Men of Harlech, on to glory!
See your banner famed in story
Waves these burning word before ye,
"Britain scorns to yield!"

'Mid the fray, see dead and dying,
Friend and foe together lying;
All around the arrows flying
Scatter sudden death!
Frightened steeds are wildly neighing,
Brazen trumpets hoarsely braying,
Wounded men for mercy praying
With their parting breath!
See---they're in disorder!
Comrades, keep close order!
Ever they shall rue the day
They ventured o're the border!
Now the Saxon flees before us;
Victory's banner floateth o'er us!
Raise the loud, exalting chorus:
"Britain wins the field!"

Men of Harlech, in the hollow
Do ye hear like rushing billow
Wave on wave that surging follow
Battle's distant sound?
'Tis the tramp of Saxon foemen,
Be they knights or hinds or yeomen,
They shall bite the ground.
Loose the folds asunder!
Flag we conquer under!
The placid sky, now bright on high,
Shall launch it's bolts in thunder!
Onward, 'tis our country needs us!
He is bravest he who leads us,
Honor's self now proudly heeds us---
Freedom, God, and Right!

Rocky steeps and passes narrow
Flash with spear and flight of arrow.
Who would think of death or sorrow---
Death is glory now!
Hurl the yelling horsemen over,
Let the earth dead foemen cover.
Fate of friend, of wife, of lover
Trembles of a blow!
Strands of life are riven.
Blow for blow is given,
In deadly lock or battle shock
And mercy shrieks to Heaven!
Men of Harlech, young or hoary,
Would you win a name in story?
Fight for home, for life, for glory,
Freedom, God, and Right!