From the certificate:
On the front
"Warning: A test certificate is not evidence that the vehicle is in a roadworthy condition"
On the back
"An MOT test pass confirms that, when the vehicle was examined in accordance with Section 45 of the Road Traffic Act 1988, it met the requirement for those items prescribed under the Act. It does not meanthat the vehicle fully meets all legal requirements or that it will continue to be roadworthy for the next year. It is your responsibility to keep the vehicle maintained so that it would always meet MOT standards and be free from other defects"
In short, the current MOT certificate is only valid unless there is reason for it to be proved otherwise i.e. you have it tested again and the latest test results in a fail. As it has been proven your car is unroadworthy even the old certificate is now invalid.
If you got the necessary work done for the new test then the old certificate would become valid without having to take it for the retest, however your old cert will expire in the month you have left on it. The reason you have 10 days to get the work done is because that is the maximum time allowed for a retest to be done on the defective parts without having to do a full test.
Don't shoot me if i'm wrong, just going on what I have been told in the past
