Hi Greg -
Bessieres was exaggerating a bit on enemy strength or had gotten hold of some inaccurate information. Von Kall had been defending Biezun and had just withdrawn towards Lautenburg. Lestoq ordered von Diericke to join him - the two bodies united on the 21st.
Total Prussian forces - 5 battalions, 10 squadrons + 5 squadrons joining between the 21st and 23rd.
General von Kall at Biezun (withdrew to Lautenburg):
Wackenitz fusilier battalion
2nd battalion of Towarcys Dragoons -5 squadrons
2 guns – horse artillery
General von Diericke:
Rüchel IR - 2 battalions
Schöning IR - 2 battalions
Baczko Dragoons - 5 squadrons
two batteries (HAB Rentzel, HAB Kühnemann)
2nd battalion Prittwitz dragoons joined later (between 21st and 23rd)
Von Diericke advanced on the 23rd in two columns as follows:
Right column (actually engaged)
2 squadrons Prittwitz hussars (Hoepfner later identifies three squadrons at the battle)
5 squadrons of Towarcys
Wackenitz fusilier battalion
1st battalion of Rüchel IR
Left Column (arrived too late for the battle)
1st battalion of Rüchel IR
2 battalions Schöning IR
5 squadrons Baczko Dragoons
3 squadrons Prittwitz hussars (2?)
2 batterys of artillery
Overall, 21 French squadrons (dragoons and cuirassiers) were engaged or immediately present and opposed 2 battalions and 7-8 squadrons between Karnyszyn and Sadlowo. The 15 squadrons of dragoons seem to have all been engaged, its not clear if the 6 squadrons of cuirassiers were engaged or not. Two companies of light infantry held the bridge at Biezun - could be 9th or 27th legere (should be able to tell by plotting the positions of Dupont's and Drouet's divisions) The French infantry was not actually engaged. The Prussian right column was cut up badly with the remnants falling back on the left column. Roget's pursuit seems to have ended when the remnants of the right column joined the left column, allowing the left column to retire in good order. I have not seen any explanation for the delay in the advance of the left column (it seems that the two columns were to have converged at Karnyszyn). The French did not continue the action after the two columns had joined and the odds were evened up. But they had accomplished their objective of turning back the advance by cutting up half of Diericke's force before it could be reinforced.
Lechartier describes the action as follows:
On the evening of the 22nd Bessieres occupied Sadlowo and Karnyszyn on the right bank of the Wkra with dragoons from Sahuc’s division. Since Bernadotte’s arrival was imminent and two companies of light infantry had already arrived, Bessieres resolved to hold the bridge of Biezun with the two companies of infantry. The bridge at Karnyszyn was left undefended to lure the Prussians into the open where the cavalry would have an advantage.
Biezun sits on the right bank of the Soldau R. A road runs northeast across the Soldau and crosses a smaller stream, the Lula and then passes through Karnyszyn. Two roads from the northeast meet at Karnyszyn, one from Chamsk and one from Dembsk. These roads passed through a large wooded area before reaching Karnyszyn. Sadlowo was about midway between Dembsk and Karnyszyn.
The morning of the 23rd, Diericke ordered an attack on Biezun. On each of the roads leading into Karnyszyn, Diericke deployed two or three squadrons. The right column was composed of 2 squadrons Prittwitz hussars (Hoepfner later identifies three squadrons at the battle), 5 squadrons of Towarcys, Wackenitz fusiliers, 1st battalion of Rüchel and 2 guns. The left column (remaining 3 battalions, 8 squadrons and artillery) was late and didn’t take part in the action. The right column debouched from the woods between Chamsk and Karnyszyn around 8 AM. The two leading squadrons of Prittwitz hussars occupied the village of Karnyszyn. They were followed by three more squadrons of Towarcys, approx. 270 infantry and the two guns.
The Prussians had barely had time to throw some parties of infantry in Karnyszyn and Sadlowo and to position its artillery when a brigade of French dragoons (Roget – 3rd and 6th dragoons from Grouchy’s division) emerged over the bridges at a rapid pace and engaged the Prussians. Roget had orders to charge and push the Prussian force back into the woods. The ground where this brigade deployed presented a kind of funnel dominated by a plateau where the Prussian cavalry formed a line. The Prussian line was supported on Karnyszyn on the right and the woods south of Sadlowo on the left. Grouchy ordered the 6th dragoons to approach the village of Karnyszyn by its main street and to turn it by the left, while 3rd dragoons would attack the line of Prussian cavalry. The 6th regiment swept into the village, where the Prussian infantry dropped their weapons, and fell on the flank of the Prussian squadrons that 3rd regiment had engaged. Most of the Prussian force from Chamsk [i.e. the right column] was captured: 500 men, five guns and two standards were captured.
Behind Roget’s brigade in support were the brigades of Margaron (Sahuc’s division), Saint-Sulpice (d’Hautpoul’s division) and the 27e dragons of the Milet brigade. These forces advanced on Sadlowo, forcing the Prussians to retire.
By evening, Sahuc’s division occupied Karnyszyn and Sadlowo; Roget’s brigade pursued the Prussians. The remnants of the Prussian right column joined with the larger left column and the force retired on Kudsburg without any further engagement and then to Soldau.
Hope this helps. If you can't identify the regiment the light infantry came from, I can dig a bit deeper.