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"And yet arrests per thousand attendees are higher at Glastonbury than at Notting Hill."

Glastonbury = 44 / ~ 200,000 = 0.22 / 1000

Notting Hill = 454 (excluding several hundred more related before the event!) / ~ 1,000,000 = 0.454 / 1000

So nope - Notting Hill is higher.