Conclusion: No real advantage in taking the car in for MOT a month earlier than the expiry - so begs the question - what's the point of it in the first place??? 
An MOT can be carried out up to one calendar month prior to the expiry date of your existing MOT certificate, whilst still preserving the anniversary of the expiry date. If you have your test carried out a month before the due date, your MOT is effectively valid for 13 months.
Works fine if it passes, but appears to leave you vulnerable (to prosecution for failed items and dodgy insurance insurance / accident status) if you're still driving around in that period until it's fixed / re-tested.
