Try closing the lock latch while the tailgate is open, then press the release button to reset it. If it dont try taking the latch apart, lube it up, refit and try again. Aside from that a mechanic might be useful.
lose bolts in the bottom bit, the boot "frame" (as opposed to the "door") get the dealer to tighten em. I had same proble, my dealer told me they could book me in "a week on thursday", I very politely (and repeatedly) informed them that I wasnt going anywhere till I could lock my car, and that if they didnt co-operate and do it there and then that they would be named and shamed on a variety of MG forums...that seemed to do the trick ;-)
^^^^^^^^^^^^Yep - as Jibbles said.....I did exactly the same on mine when it happened. I loosened the bolts on the plate, moved it "up" a few millimeters so the door would meet it slightly higher up and then tightened the bolts. Worked fine ever since and that was over a year ago! :up: