maybe you could let vosa know this as the mot seminar i went on earlier this year this was one of the topics discussed as there seems to be a little confusion amongst people on this, from the horses mouth so to speak.
Every new test has its own unique test number and always superseeds any previous test, the mot test is set at 12 months that is the maximum if you present a vehicle that is not roadworthy earlier than that it fails from that date and likewise unless it was within the last 30 days till expiry the new test would start from the day presented regardless of wether it had say 10 months remaining.
If you are a tester have a look on the vts under scheme proceedures.