It actually arrived late afternoon the day before yesterday (although I didn't see it happening). I haven't moved it I know if I'm going to I should do it soon, but nothing nasty seems to be happening. It's really quite well hidden (not by the box - by a wall and the fact that it's below ground) - I'm probably the only person who's seen it.
I know that magpies mate for life, but hadn't thought about that until you mentioned it - I just hope this one didn't have a mate.
I'm not too traumatised by the whole thing - probably because I didn't see it happen. Was more distraught when I saw (at intervals) a live, then dead, then eaten-by-seagulls baby pigeon.