If a dangerous defect is recorded I could see you may have a problem if something happened, as you had been made aware of it. My understanding is that driving an unroadworthy vehicle on the road can result in a prosecution whether it has an MOT or not.
A Traffic Officer told me that they cannot access test results they can only check if a valid MOT is in force, so they would not be aware of the fail anyway.
I am not convinced that a fail overrides an existing Test, and a phone call to Vosa is not guaranteed to deliver an accurate response, there are many examples of Vehicle Examiners having differing views.
