Again I have just come back from a refresher course and asked this very question. The current MOT certificate remains a legal document until its expiry. The problem arises when the customer presents his vehicle a month before the test and it fails, now that he has been informed of defects he may jeopardize his insurance knowingly driving a defective vehicle, his current MOT certificate still remains in force and legal for re-taxing etc.

Example you could drive away from a brand new MOT and the next day blow a brake light bulb according to your highway code you should check the operation of your lights daily until you get it fixed you could be knowingly / unknowingly be driving a defective vehicle. The vehicle can become defective at any time its up to the authority's or insurance companies to make their decision whether there has been negligence in maintaining the vehicle, whether this is contested in a court of law or just a ticket from the police to get it repaired. We must stress the importance to our customers about keeping up regular servicing and maintenance on their vehicles because if faced with a court case a service history may go a long way!
John Kennard.