By 6pm on 17 April, the withdrawal "was well underway". There was a rearguard in place consisting of two infantry companies along with a troop og guns. The New Zealand 5th Brigade were on the main road, just to Pharsala. At Pharsala, they moved to the eastern road.
The Servia Pass had no infantry fighting on 17 April. The only fighting was artillery firing periodically. The New Zealand guns were pulled out "later in the afternoon". The New Zealand infantry were loaded into trucks after 8pm.
A famous New Zealand officer, Howard Kippenberger, was put in charge of demolition. He was to leave the pass by "3am on 18 April". The 19th New Zealand Battalion was out of there "before midnight". Their vehicles were traveling dispersed and were moving quite fast to the south. The New Zealand 18th Battalion was having problems. By 3am, two of their companies had not yet reached where Kippenberger was waiting to start demolition. One of the two only arrived right before 4am while the second company arrived soon afterward. At that point, Lt-Col. Kippenberger ordered the senior engineer to "demolish the road". Once the demolition had started, there were "cries" from New Zealand troops "on the other side". The engineers waited for the stragglers to arrive. Kippenberger only ordered the "final demolition" at 6am.
After the demolitions, the 6th Brigade Group now covered the road from Elasson. You now had Savige Force near Kalabaka. Savige Force was visited by Liaison officers four times. As the historia said, the bridge had been destroyed twice and repaired twice. After the Greek commander, General Papagosk thought the British should leave. When Churchill heard that, he thought that they had to leave.
General Wavell agreed that an evacuation was inevitable, but thought that Wilson should not rush the withdrawal. This is based on the account in "Greece, Crete, and Syria", by Gavin Long.