I don`t get them done very often (perhaps once a year ;-) ) - is it a month prior to the old one running out that you can put it in again with the old certificate to get it post-dated (i.e. 13 months) ?
If a car fails the MOT, are you still legally allowed to drive it until the the old certificate runs out ? (ISTR they were changing the way they were done, and wasn`t sure how this aspect panned out now)