Has a national treasure morphed into a sheep in sheep's clothing or a human shield? I am referring to a woman who has hurtled back into the headlines for the first time since her Gang of Four deserted Labour in 1981 to form the Social Democrats. Now Shirley Williams is being accused of another act of betrayal – not just supporting the Government's unpopular Health Bill but becoming its chief proselytiser. In so doing she has given political cover to Nick Clegg, who still had to watch yesterday as Liberal Democrat delegates lined up to denounce the reforms.

The opprobrium heaped on Williams has been a mix of anger, sorrow and petulance. Another sister of the Sixties, Polly Toynbee, declared that her erstwhile friend had mistaken political discipline for sheepdom – a serial Liberal Democrat habit in government. One of the most disappointing comments of recent days came from a Labour grandee. When Lord Hollick called Williams "the Coalition's Lady Haw-Haw", not only was he displaying bad taste, he was ignoring his party's own record in bringing the private sector into the health service.

The issue at stake is far more important than the personal spats. Clegg has moved from initial nonchalance about Andrew Lansley's plans, to promising an overhaul of the Bill, to achieving some change, but not nearly enough for many. The saga provides another salutary lesson about expectation management within a coalition.

How much influence is enough to trade against the guilt by association with the Conservatives? Clegg knows that is impossible to answer. He responds that something is better than nothing. But will the pitch "we've made the Tories a bit less nasty" wash at the next election?

Ed Miliband cannot believe his good fortune at seeing David Cameron tie himself to the one issue that remains toxic for the Tories. Labour thinks it can sit back and denounce from the purity of opposition. In the short term, as long as voters fail to see Labour's double standards on its NHS record, this approach will reap dividends. In the long term it will remind voters that the party has yet to grow up. Labour continues to regard Clegg's decision to jump into bed with Cameron as the ultimate act of treachery. One wise head is Jack McConnell, who served as First Minister in Scotland, working with the Liberal Democrats in government there. "There's a public willingness to go with politicians who are seen to want to compromise and to work together," he told me in 2010.

In an emotional address, Williams denounced her Labour critics for "tribalism trumping truth". I'm not sure she's right on the truth part. This Bill, even while improved, remains deeply flawed. But on the narrowness of the tribe, this grand dame of the centre-Left has nailed it in one.

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