While there are valid concerns about the role of the audit firms, the fact is that it is impossible for accountants to spot or forecast problems at project based companies, when project staff, project managers or company directors can't spot the problem themselves.
Take an example of a (oversimplified, imaginary) project bringing in £1m per month over 5 years, with a 10% profit margin. At the end of month 1, there's a £100k profit sitting on the projects books, does the company declare that in their financial results? If they don't the £1m turnover will appear in the books with a zero profit, which will not please shareholders, the City or their bankers. If they do there's no buffer against future cost overruns, that £100k has gone never to return. And of course cost overruns only becoming apparent part way into a project is an all too familiar feature. Say later on there's a £5m overrun, not a disaster in itself - the project overall still breaks even, but if the earlier profit has been declared, it will mean booking a loss and perhaps issuing a profit warning.
The situation at Carillion was even worse as they were running at 3% profit margins and the government apparently generously gave them large up front stage payments as an incentive to lower the price. It is believed cost overruns on only 4 contracts bought them down.