Your MOT is valid until its expiry date even if in the meantime your
car fails the MOT - the original pass is still valid. In fact you can have the Test done up to a calendar month before the expiry date and have the new MOT forward-dated to the expiry date of the existing certificate. So if you have your car Tested during that month and it fails, this gives you time to have the repairs carried out and the car re-tested before the expiry of the original certificate and you are not disadvantaged as a result. - MOTT.
From a place
"MOT Test information for the motoring public, the Testing Station owner, Authorised Examiner, Nominated Tester, repair and service technician"
http://www.motester.co.uk/mot-qanda3.html