From the AAIB report:
NATS reported that, since January 2004, there have been 21 occasions when aircraft departing London City Airport have climbed above the step altitude of 3,000 ft published in the SID. A third of the incidents led to a loss of ATC separation. Evidence from London Stansted Airport showed that removing the step-climb element of the CPT/BUZ SIDs led to a reduction in the number of aircraft that climbed through the first cleared altitude after takeoff. There were 12 ‘level busts’ reported on the SIDs in the 24 months before the end of 2005 when the step-climb was removed. There were 5 level busts in the following three years and seven months.
That's far more history than I had imagined. I take back my somewhat tongue-in-cheek comment about the ANSP moving quickly when it feels the need. Clearly the ANSP felt they knew best at the end of 2005 and didn't feel much disposed to heed a further need until prodded sharply by the regulator.