How long does it take for the 10 penny to drop with Labour MPs

Gordon Brown Budget speech to the House of Commons in March 2007 headlined on the reduction of income tax from 22p to 20p. At the despatch box, he explicitly failed to mention the withdrawal of the 10p rate, for obvious reasons. However, the text of the Finance Bill, which was published at the same time, laid out these plans and the change made newspaper headlines in subsequent days for the apparent sleight of hand. Indeed, today's Daily Telegraph has a "I told you so article" here

The introduction of these changes has only recently taken place, prompting a backbench uprising amongst Labour MPs who are expressing concern about the disproportionate effect on low paid workers.

So here's my question [which many others are also asking] - why has it taken so long for these MPs to spot the impact of this change when it was clearly outlined 12 months ago and widely reported?

Were they negligent in scrutinising the contents of the Finance Bill and hence failed to spot it? If so, do they have similar problems with other legislation that they nod through? Or were they previously too concerned about raising the issue for fear of Ministerial reprisals? Or is it only when constituents start kicking their MP that they begin to care?