Thanks for all your input.... I thought I would see what Canon had to say re the problem....I sent them the images and details...their technical services team at Canon suggest that I take the camera to their Service centre for repair. After viewing the images, they think there is something intermittently obscuring the sensor... I will do a few more tests tonight just with the standard lens and see if I can reproduce the fault....And guess what!!! I'm about a month and a half out of warranty!!
.....maybe if I plea my case that this is the first time I have used the camera with time exposures..... (which is true)...they may be a little sympathetic...
... fingers crossed !!