Through a friend in the hospitality business we got free VIP tickets for ZZ Top at MK Bowl - we even parked next to their helicopter! I've been to the Bowl quite a few times and never saw anything as disappointing as ZZ Top (who I didn't like much anyway).
I think the capacity of the Bowl is around 50,000 - and I would estimate that 15 to 20,000 turned and left after the final support act finished, Sir Bryan of Adams.