At a guess I reckon the plug had been over tightened, resulting in cracks around the thread housing, we could have taken it out and mucked around with it, but if this works (and so far so good) its good enough. If it doesn't I'll get a new tank.
Now we've had it out once getting it out Ahmadinejad won't be an issue.
And yes Jim, that is an EX drain plug!
Did you notice how auto correct found Ahmadinejad instead of "again?" Weird!