At the end of the day an MoT is not a guarantee of a vehicles safety. The idea of allowing test to be carried out a month in advance is to stop excuses for driving without valid certificate. I was going to write a long answer but then chose to google and found this which sums up and (as much as t'web can be trusted) confirms my belief rather well.
OK Here is now THE DEFINITIVE ANSWER! I just spoke to VOSA on 0300 123 9000 and here is what he said.
"An MOT certificate is valid until the date of expiry. A new test will not invalidate the current MOT certificate. However, the "Fail" test result will show on the DVLA data base along with the "Pass" test result (ie any search on the DVLA data base will show both a pass and a fail result)."
He also raised the following important point!
"The police can access the data base and if while following your car an officer checks the MOT status, he would see a "Fail" and would be likely to pull you over to see if the repairs have been done or if the car is not roadworthy. You might then be prosecuted"
So here is my answer :-
YES! The first MOT test certificate IS still valid so you can still legally use the car on the road, insure it, tax it etc.
BUT! The "Fail" will be recorded on the data base and you seriously risk prosecution if you continue to drive a car with a fault which renders it not roadworthy.
I know others have eluded to the same advice but mixed with the "opinions" it is difficult to sort the wheat from the chaff.
Source:
http://www.dft.gov.uk/vosa/ and phone call to 0300 123 9000[/font][/size]