Hi, Dick -
Might you like to post the lyrics and tell us a little about the recording?
nostalgia.wikipedia.org has a pretty good summary of the battle:

The Battle of Bosworth Field was fought on the 22nd of August 1485 when Richard III, a Yorkist, fought a pitched battle with the Lancastrian contender for his crown, Henry Tudor. Henry Tudor had landed in Wales with a small force in an attempt to claim the throne of England. Richard III had fought similar battles with Lancastrian usurpers in the past, but this one would be his last. Richard III was killed on the field and the battle proved to be decisive in ending the long-running mediaeval series of English wars known as the Wars of the Roses.
Henry Tudor's victory in this battle led to his being crowned as Henry VII, [beginning] the long reign of the Tudor dynasty in England.

Ballad of Richard III in the Digital Tradition also tells the story.

From Shakespeare's Richard III, 1591/2:

CATESBY:
Rescue, my Lord of Norfolk, rescue, rescue!
The king enacts more wonders than a man,
Daring an opposite to every danger:
His horse is slain, and all on foot he fights,
Seeking for Richmond in the throat of death.
Rescue, fair lord, or else the day is lost!

KING RICHARD III:
A horse! a horse! my kingdom for a horse!

CATESBY:
Withdraw, my lord; I'll help you to a horse.

But could we have the lyrics to your song, Dick?
Thanks.
-Joe-