Red faces all round

According to The Observer, the Tories have been balancing the electoral scales in their favour:

David Cameron has been accused of an unjustifiable bid to “buy the general election” as it emerged that ministers have quietly slipped through an unprecedented hike in the amount that parties can spend during the campaign.

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The change to the law on candidates’ election spending, passed without parliamentary debate, was made despite a direct warning by the commission against such “excessive spending to prevent the perception of undue influence over the outcome of the election”.

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The Observer has learned that ministers changed the law through a statutory instrument, the terms of which were not debated in the Commons, and which is more usually a vehicle for consensual changes in the law. A Labour source said that the move had not been spotted by them at the time and so they missed the chance to force a vote in the Commons.

Leaving aside the iniquitous behaviour of the government, why did Labour not spot it at the time?

The statutory instyrument would have been subject to negative resolution procedure, which means that, after being laid before the House (and published on the order paper), it automatically becomes law after a certain period, unless it is prayed against (i.e.an MP formally objects), in which case the instrument has to be debated.  So, was no-one in the Labour Party (or in other opposition parties) designated to keep an eye on the flow of statutory instruments coming before the House?  If not, why not?