Is the fare about £500 one-way? I found a table on a website after doing a Google search for M12W United or M12W fare, and it is the third result from the top (M12W is apparently both CO and LH-sold). Sounds like definitely coach, BUT, because you are getting a business seat, it perhaps might be a mistake.

Usually in the case of mistakes like this, I've heard of airlines honoring the seats.

YMMV.