Broadcasters might be forgiven for not having shown Tuesday night’s Scottish independence debate live on television south of the border – even David Cameron was not sure he would watch the whole event.

Hours before it started, Downing Street said the prime minister would “catch up” on the debate from his holiday in Portugal. But advisers would not say if he intended to watch it live or in full.

The prime minister’s relaxed attitude to the clash reflected a feeling in Westminster that it was unlikely to change what many believe is a hardening attitude against independence in Scotland.

“This is not going to be a game changer,” said one Whitehall official involved in the pro-union Better Together campaign. “Salmond’s had his chances to turn this on its head and he hasn’t managed it so far. There’s no reason to believe this will do it.”

That belief appeared to have been borne out by a Guardian/ICM poll published after the debate, which found that anti-independence campaign leader Alistair Darling had confounded expectations by scoring a narrow victory.

The normally polished Mr Salmond appeared to lack some of his habitual charisma in front of a studio audience, which booed him on his answers about what currency Scotland would use.

The “No” campaign has also been buoyed by opinion polls that have consistently given it a double-digit lead over its “Yes” rivals since the outset of the campaign.

Many investors in the City have stopped watching the Scottish independence issue so closely in recent weeks, convinced by polls and betting odds that overwhelmingly point to a “No” victory that there is nothing to worry about.

But Rory Stewart, Conservative MP for Penrith and the Border, remains concerned about the perceived lack of interest in the debate south of the border. “In Canada, a million people marched to ask Quebec not to leave. There is no such movement in the rest of the UK.”

However, Gemma Doyle, a Labour shadow minister and MP for West Dunbartonshire, insisted: “My colleagues in England are interested in this – it is the biggest political event we will have this year.”

She added: “[Mr Darling] needs to rise above the fray and be statesmanlike. That’s his strength.”

Colleagues acknowledge that although the former chancellor’s measured tone during Tuesday’s debate appeared to help him defeat the first minister, it was unlikely to enthuse a new raft of campaigners, inside or outside Scotland.

The “No” campaign continues to labour under the charge that it is almost entirely negative and fails to offer any positive vision of the future for Scotland within the UK. Mr Salmond dubbed it “Project Fear” during the debate.

It may have been punctuated as much by a stuttering internet connection as fiery political debate, but for those watching in Westminster, Tuesday’s clash between Alex Salmond and Alistair Darling was gripping viewing.

“This is a brilliant debate,” said Chris Deerin, the political commentator.

“The best political debate we’ve seen, better than the 2010 leaders’ debates or Farage versus Clegg,” was the verdict of one member of a business audience watching in Central London. The person did not want to be quoted by name, many businesspeople are still terrified of saying anything on the independence campaign, no matter how apolitical.

For many of those on the pro-union side, Alistair Darling’s passion was both surprising and the defining element of the debate. Mary Macleod, the Conservative MP, said: “Some of the questioning from Alistair Darling was extremely effective in trying to dig into what answers Alex Salmond really has.”

Another pro-union campaigner said: “This felt like a score-draw from here. But given the pressure was all on Salmond, that was enough.”

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