Far be it for me to contradict an expert. I remain just an enthusiastic amateur. I had not answered as this was difficult without more information, and I feel that is what our expert felt too. It was a good statistical guess, being in an urban park (rats), and the most common (most likely choice). The slenderness worries me, especially as the word 'very' was used. I have encountered snakes in parks that had water (rock waterfalls and fish) that were keelbacks and racers. Showing my ignorance because if I cannot identify a slim, fast snake I put it in the racer category, knowing only the mountain racer in HK. Mountains not normally being found in parks. Keelbacks like fish and are common, I have seen them close to urban areas. The colour might not be right though. I fear, mystery unsolved. But statistically the expert should be correct.