Our current auditor disagreed with the approach taken by the previous auditor which created a large amount of work which costing time and effort – how does this work efficiently or even reliably?

Once appointed auditors are independent and the decisions that they take are down to their professional judgements. There may be differences in judgement between years by individuals from the same firm as views change or where more evidence becomes available. We are reviewing the circumstances that you describe for the next contract to see if there is a way to resolve the issue as equitably as possible.