Well I spoke to the DVLA today and they've got the message from the garage. They just asked me to send my MOT cert to them and they'd re-issue a new one with the right details on it.
Interestingly enough, I had a direct dial number to the "MOT Mismatch Team" who apparently deal with this stuff full time! I guess it's not all that common then.. mind you, when they took my ref number they asked if it was a mismatch between my Porsche 959 and a Mini Cooper!!! Bit of an obvious mistake that one you'd think!
MY14 Space Grey F11 530d M-Sport