The mottled appearence of your image is cause by something called noise. It's more prevalent at higher ISO settings. It is possible that your camera automatically increased the ISO setting when it sensed that the surroundings were darker.
A flash only works on objects that are fairly nearby, and even so, the lighting is a little harsh and unnatural. The best way to reduce noise in images is to take note of camera settings before taking the shot. It is recommended that you manually set the lowest possible ISO on your camera. Turn off the flash.
Find a night scene mode (or if you're more advanced, manual mode) on your camera. If there isn't one, then stay in the regular Program or equivalent mode. Do not use Auto mode since you will lose control over flash and ISO setting. Place your camera on a sturdy surface or tripod, frame your shot, set the camera to self-timer mode and trigger it off that way to reduce chances of camera shake.
If you must take a low-light photo handheld and want to reduce noise levels later, there are a number of noise reduction software on the web. Try downloading a copy of programs such as Noiseware, Noise Ninja, Neat Image...etc. There must be more, but I am not familiar with this.