After a weekend of fluctuating fortunes for the SNP, a senior figure in Scottish tourism and the arts has joined the business battle for Holyrood, saying independence would provide an opportunity rather than a threat.

Donald MacDonald, who operates businesses in Scotland, England and Ireland, with England as the major market, rubbished Labour's argument that Scottish commerce would be cut under independence.

His intervention came at a time when there was good news and bad news for the Nats. While the second full week of campaigning began with a boost - an opinion poll in the Mail on Sunday gave them their biggest ever lead over Labour with 12 points in the constituency vote and 11 points in the regional vote - it was tempered by revelations about the private life of one of their leading politicians.

Angus MacNeil, MP for the Western Isles and the party's hammer of Labour over cash for honours, was forced to issue a humiliating apology for a "wrong and stupid" lapse of judgement - a drunken night two years ago which saw him end up in bed with two teenage music students. Although there was no suggestion that sex took place, what he called the "foolishness at a post-ceilidh party" in Shetland took place while his wife was heavily pregnant at home on Barra.

The revelations came as Alex Salmond prepared to visit the Western Isles today. Party sources claimed police were investigating a wave of "dirty tricks" against Mr McNeil.

Labour also had its troubles, with a newspaper claiming that contenders were lining up to challenge for the leadership if a poor election result forced Jack McConnell to step down.

Health Minister Andy Kerr had been able to issue a total denial but Wendy Alexander, the Enterprise Minister, had been unable to do the same because she was with her family on Mull.

Yesterday, she told The Herald: "This is ill-informed, unsourced and malicious spin, designed to benefit the SNP. Like all Labour colleagues, my focus is on winning the maximum support for Labour in May."

Mr MacDonald writes in a letter to The Herald today: "Having developed with other Scots a number of businesses in Scotland for whom England is now the largest market, we have no fears about these markets in the wake of the independence. Just consider the success of many Irish products despite the Troubles."

His contribution is backed up with a £20,000 donation to the SNP, aimed at promoting its campaign message to an estimated 120,000 small businesses that it could remove them from business rates requirements or reduce bills for 30,000 more.

The comments follow an article in The Herald last week by Ben Thomson, chairman of investment bank Noble Group, and echo the financier's argument that independence is "an opportunity and not a threat".

Mr MacDonald was one of five people to set up MacDonald Hotels in the 1990s, though he split from them to set up MacDonald Orr, which runs the City Inn chain. He is joint chair of Caledonian Brewery and was president of Edinburgh Chamber of Commerce. As chairman of the Scottish Chamber Orchestra and a board member of the Edinburgh International Festival, he is also keen for an independent Scotland to replicate the Finnish success in funding the arts.

His letter says: "Speaking to many people (not only businessmen) their minds are turning to consider the benefits of independence, where previously agnostic or hostile, and are now recognising that Scotland is underperforming and needs cultural change".

A spokesman for the party said: "The SNP's economic strategy, Let Scotland Flourish, has struck a chord with small businesses and the small business rates relief scheme in particular will give the country's smallest businesses an opportunity to thrive which will benefit towns and cities across Scotland."

Labour has commissioned Populus research company to test opinion among Scotland's biggest companies, and it found large majorities of business leaders think independence would cost Scotland, 88% think it would make it more difficult to work with different currency and regulatory systems from England.