Labour on the London Assembly seem as punch-drunk as the party nationally, a weakness that the Mayor of London Boris Johnson happily exploited at his City Hall Question Time.

There was, for example, the glee with which the Mayor leapt upon Tom Copley’s ungainly triangulation over social housing in which the Labour man appeared to rail against rent cuts for impoverished tenants.

A more confident Labour team would done a better job skewering the Mayor on his weak flank – woeful affordable housing levels in new-builds, a policy characterised by generous, almost naive, pragmatism.

“25% of something is better than 50% of nothing,” said Boris. But he admitted that some developers were gaming the system, hiking up the upfront costs and playing down the value of the finished article.

Recently we have seen Enderby Wharf passed with 16% affordable and Millharbour Urban Village booted out with 25%. The core strategies suggest 35% but developers can find leeway in the dark arts of the "viability assessment".

• Also online: Why affordable housing falls at the first hurdle

Even councillors don't get to see these reports and there is little confidence in the final result, even though they are independently vetted. There is a move to address these viability assessments but councillors are being given the unpalatable choice of "confidentiality = developers' openness" or "transparency = developers' obfuscation".

“Some developers are getting away with it,” Boris conceded.

Earlier he said that homes would be built regardless of policy because, well, it’s boom-time in London. Surely (as no-one from Labour pointed out) that is exactly the time to apply stringent affordable housing levels and see who blinks first.

These sites are rich with opportunity but they’re also a one-time deal. Once set, these levels are locked in for life.

The Tories used to attack Labour for failing to fix the roof while the sun shines. Now the mayor’s handing out roofs like fried chicken flyers and we’ll dream of the days when they could have been ours to mend.