hmm unless you were selling it on and wanting to have the proof in your hand to show a buyer, then i can't think of why you would need proof of the MOT. It's is the 'system' that your car is MoT'd, so you can even buy a tax disc online, as long as you have your reg to hand.
any garage would be able to check your car in the same way you could re the mot.
I just bought a car and promptly managed to lose its MOT certificate. I've looked up the details online b/c I have the document reference number from the V5C registration certificate so I know when my current MOT expires. Am I legally obliged to actually have an official piece of paper though or will I need it in order to get my next MOT done? I now have a printout of the online DirectGov page and am wondering if that's all I need.