A couple of thoughts....
1. The 450D is not included in the list of compatible cameras on the Jessops website and, in fact, the 40D is specifically excluded, so are you sure the 450D is intended to work with this flash unit? Since the website makes no remark about the 450D, one way or the other, who is to know?
2. To use the flash will require the shutter speed to be no faster than the max sync speed of 1/200. If your camera settings have a shutter speed faster than this in order to achieve a correct exposure (e.g. 1/1000), when you turn the flash on it will force the camera to 1/200. That may cause an overexposure of the ambient light on its own, never mind any extra light from the flash. What are your settings for shutter, aperture and ISO with the flash turned off? What do they become when you turn it on, if they change at all?
3. Even though the flash may have no controls on it, the camera should surely have a Flash Exposure Compensation control (FEC). This should allow you to dial the flash strength up or down to control the flash output. What is your current FEC setting? Can you adjust it?
I don't have a 450D, only 30D/40D/50D, so I am making some assumptions here about the features of the 450D..