sddsddean's answer is correct but I've seen plenty of cases, in the paper, where drivers have been charged with having no MoT for the vehicle they were driving but not charged with having no valid insurance at the same time. This suggests that, unless the vehicles is involved in an accident, the absence of valid insurance might be overlooked by the boys in blue (assuming, of course, that the driver actually has an insurance document).
Chris
PS: Now I've read this thread, I think I ought to look up the date on my MoT document. Now I come to think about it, I've got a horrid feeling . . . :-(