By the end of Wednesday 20th September, the remaining men of the 1st Airborne Division were assembled into a firm defensive pocket to the west of Oosterbeek. The Western side of the Perimeter was well defended by the intact 1st Border with mixed units of Glider Pilots and Royal Engineers plugging the gaps. In the north, the 7th King's Own Scottish Borderers dominated the position, with the Reconnaissance Squadron to their rear and their left flank held by the Independent Company and part of the 4th Parachute Squadron. All of these largely glider-borne units came under the command of Brigadier Hicks.
The initially almost undefended Eastern sector, generally consisting of parachutists, began to take shape under Brigadier Hackett. In the extreme south was the Lonsdale Force, and behind them were the guns of the 1st Airlanding Light Regiment. Between the Lonsdale Force and the River Rhine there lay a quarter of a mile of undefended and featureless land, however this could easily be covered by machine-gun fire during the day and by patrols at night. Guarding the important Utrechtseweg-Stationsweg crossroads in the centre of the Eastern defence was the 10th Battalion. To the north of them, the 156th Battalion took up positions along the Stationsweg when they arrived in the Perimeter during the evening. In between all of these units lay two Squadrons of the ever-reliable Glider Pilot Regiment and the 250th Light Composite Company.
Divisional Headquarters was established in the centre at the Hartenstein Hotel, and hereabouts was the reserve force consisting of several units of Glider Pilots and assorted other groups. Two hundred yards south of the Hartenstein were tennis courts where German prisoners of war were guarded by the Divisional Provost Company and Glider Pilots. Major-General Urquhart felt confident that the 1st Airborne Division could hold this position until ground forces arrived on the southern bank.
The 1st Border, dug-in along what was to become the western edge of the Perimeter, were subjected to numerous attacks throughout the day, having easily beaten off light probes by Kampfgruppe von Tettau on Tuesday. Their positions were shelled throughout Wednesday, and were followed up by several probing attacks during the morning. It was during this bombardment, at 08:30, that 1st Airlanding Brigade Headquarters received a direct hit and four officers were killed and many other staff wounded. Despite being in amongst this group at the time, Brigadier Hicks escaped injury.
At 10:00, "A" Company was attacked by infantry and a self-propelled gun, but these were comfortably repelled. "C" Company were repeatedly harassed throughout the day and they were beginning to struggle; the woodland in which they were fighting being of a difficult nature to defend, and observation of the enemy's movement was barely possible. At 15:00, they were heavily assaulted by infantry supported by a self-propelled gun and two tanks mounting flame-throwers, and several of "C" Company's positions were lost. These were soon retaken, however, and the infantry were forced back when an anti-tank gun destroyed the self-propelled gun.
In the north of the Perimeter, the 7th King's Own Scottish Borderers suffered little more than harassment from enemy snipers, machine-guns, and shelling from a self-propelled gun. A notable success of their day came when a patrol managed to attract the attention of two German tanks and successfully lured them on to one of the Battalion's anti-tank guns, which destroyed one of the vehicles and prompted the other to flee.
Major-General Sosabowski's 1st Polish Independent Parachute Brigade Group was still in England at this time. They were to have been dropped south of Arnhem Bridge on Tuesday 19th, but bad weather had forced the cancellation of this lift. In view of the new situation, Urquhart arranged for the Polish Brigade to land near the village of Driel, a mile to the south of Division's positions on the other side of the Rhine. Once here, they were to make use of the Driel-Heveadorp ferry crossing, which ran into the base of the Oosterbeek Perimeter, to bring the entire Brigade across. The Poles were to have arrived on Wednesday, however poor weather conditions again necessitated the postponement of their lift for a further twenty-four hours.
The RAF mounted another re-supply operation on this day, using one hundred and sixty-four Stirlings and Dakotas. On the previous two days the aircraft, not knowing any better, dropped their loads on zones that had already been overrun by the Germans. On Wednesday, however, Divisional Headquarters had been able to alert them to this fact and arranged for the supplies to be dropped inside the Oosterbeek Perimeter. Unfortunately, the situation had not been made entirely clear to the RAF and, to avoid congestion over the new dropping zone, thirty-three aircraft released their supplies over two miles away on LZ-Z, straight into German hands. Once again the formation was met with heavy anti-aircraft fire, and due a combination of the thickly wooded terrain around Oosterbeek and smoke obscuring their visibility, the crews had great difficulty in identifying the correct dropping point and so released their loads where they thought best. This method proved to be moderately successful with approximately half of the supplies ending up in British hands. The cost, however, was a heavy one, with twelve Stirlings and two Dakotas shot down. 196 Squadron lost six of the seventeen Stirlings it had committed to the lift.