The report shows that 17 per cent of completed projects have run over budget by an average of £293,000, and 6 per cent have required an average of £176,000 of extra cash to meet cost increases.
The reasons for the delays, the report says, have been down to poor planning, 'unanticipated events during construction' and a 'lack of public management skills' by applicants.
This has led the NAO to recommend that more should be done to assist projects, specifically reducing administration burdens on applicants, speeding up the application process and monitoring projects more closely.
Head of the NAO John Bourn said: 'The Heritage Lottery Fund has done valuable work in preserving the UK's heritage and promoting access to it. However, it needs to do more to simplify its procedures, to provide better support to applicants and grant recipients and to reduce time and cost overruns.'