An old Flight Sergeant under whom I was privileged to work was on 205 Sqn at Seletar at the time. He said that when the squadron was ordered to leave for Batavia (jakarta), all the NCO aircrew were left behind and replaced with officers from around the station. He along with these aircrew NCOs and all the ground crew were turned over to army command and manned defence lines along the Thompson Road. He also said that a couple of Hurricane squadrons had been down at Kallang, to give them a chance to climb to operating height, but these were withdrawn to Sumatra at around the time the 205 aircraft went to Batavia (Jakarta). He and I were both posted to Changi at the same time and he was reluctant to go. I spoke to him again some time later and he said that he was enjoying his tour and that Changi now looked nothing like it did when he was "helping" to build the runway that were now the Eastern & Western dispersals, so it wasn't bringing back any bad memories. In 1942-45 it was a building site without any grass or trees to be seen, now it was almost a garden.
Hurricanes in Singapore? The record indicates about a hundred of them. They just weren't well deployed. You need height to bounce enemy bomber formations and they didn't have the benefit of "Home Chain" Radar to scramble in time for a proper interception. Had they been based in Sumatra with radar early warning and distance to climb before engaging the enemy it might have been a different story.
More to the point, had they been at the northern airfields with radar support in early December 1941, they may have prevented the Japanese sinking the PoW and Repulse which would have at the very least given our army the time to enter Thailand and put up a direct opposition to the landings.