It's one of those weeks where you feel London's arts scene could end up totally overwhelming you if you're not careful. Strap yourselves in, there's masses going on... We'll start with outdoor opera. When was the last time we told you about watching world-class opera for free? We can't remember either. So, head to Trafalgar Square or Canary Wharf on Thursday and enjoy Verdi's Don Carlo free from 6pm on the big screens. There's......