I believe the Sharpe book refers to the 1812 Battle of García Hernández. A heavy cavalry brigade, made up for the 1st and 2nd King's German Legion Dragoons, broke two French squares. The British report of the breaking is that the French waited too long to fire at the charging dragoons resulting in a wounded horse crashing into the French square. The horse opened up a gap in which the remaining cavalrymen rode into. The French square dissolved with most of the men surrendering. The dragoons then attacked another nearby French square which was panicked after seeing the first square break. Their square broke with men either fleeing or surrendering to the dragoons.

An in-depth account:

"Captain Gustavus von der Decken, who commanded the third or left squadron of the regiment, seeing that if he advanced according to the order given, his flank would be exposed to the fire of a dense infantry square, formed the daring resolution of attacking it with his single squadron.

This square stood on the lower slope of the heights, and obedient to the signal of their chief, the German troopers advanced against it with order and determination, while deafening peal of musquetry from the enemy greeted their approach. Arrived within a hundred yards of the point of attack, the gallant squadron officer, struck by a ball in the knee, fell mortally wounded, and lieutenant von Voss, with several men and horses, were killed; but instantly, captain von Uslar Gleichen who commanded the left troop, dashing forward, placed himself at the head of the squadron, and re-animating his followers by words and example, while another shower of bullets carried destruction among their ranks, the intrepid soldiers forced onward, and bringing up their right flank, appeared before the enemy's bayonets on two sides of the square.

The two front ranks, kneeling, presented a double row of deadly steel, while in rear of these, then steady musquests of four standing ranks were leveled at the devoted horsemen. At this critical moment, when the sword was about to be matched against the fire-lock, and the chivalrous horseman against the firm foot soldier - when victory hung yet in equal scales - an accidental shot from the kneeling ranks, which killing a horse, caused it and the rider to fall upon the bayonets - gave triumph to the dragoons!

For a path was now opened, and the impatient troopers rushing in amid the blazing fire, while men and horse fell fast before the musquets of the French infantry, their firm formation was destroyed, and the whole battalion were either cut down or taken prisoners.

Captain von Reitzenstein, who commanded the second squadron, seeing the success which had attended the daring onset of his comrades on the left, and being also impeded in his forward movement by the difficulties of ground, decided upon following up the discomfiture of the infantry, and attempting the second square, which stood on the edge of the heights. He was recieved with a steady and destructive fire, by which lieutenant Heugel was killed, and lieutenant Tappe severly wounded; but the moral force of the French infantry had been skaken by the fearful overthrow which they had just witnessed, and some timid individuals leaving their ranks, Reitzenstein rushed in with his ready followers; the square broke, and the greater part of the battalion was cut down or captured.

A third square was instantly formed by those few who had escaped from destruction, and some cavalry came to their support. Against these captain Baron Marschalck led the third squadron of the second regiment, and, being joined by the left troop of the second squadron under lieutenant Fumetty, charged and dispersed the evemy's cavalry; then riding boldly at the infantry, broke and completely overthrew them."

History of the King's German Legion - North Ludlow Beamish 1837