The preparation of financial statements under IFRS requires management to make judgements, assumptions and estimates that affect the application of accounting policies and the reported amounts of assets, liabilities, income and expense. Actual results may differ from these estimates. Assumptions and estimates are reviewed on an ongoing basis and any revisions to them are recognised in the period in which they are revised.

The following items are those that management considers to be critical due to the level of judgement and estimation required:

Revenue and profit recognition

Recognition of revenue and profit is based on judgements made in respect of the ultimate profitability of a contract. Such judgements are arrived at through the use of estimates in relation to the costs and value of work performed to date and to be performed in bringing contracts to completion. These estimates are made by reference to recovery of pre-contract costs, surveys of progress against the construction programme, changes in work scope, the contractual terms under which the work is being performed, including the recoverability of any unagreed income from variations and the likely outcome of discussions on claims, costs incurred and external certification of the work performed.

Management continually reviews the estimated final out-turn on contracts and makes adjustments where necessary. Based on the above, management believes it is reasonably possible, on the basis of existing knowledge, that outcomes within the next financial year that are different from these assumptions could require a material adjustment.

The Group has appropriate internal control procedures over the determination of each of the above variables to ensure that profit take as at the balance sheet date and the extent of future costs to contract completion are reasonably and consistently determined and subject to appropriate review and authorisation.

Impairment of goodwill and other non-current assets

Goodwill is tested at least annually for impairment, along with the intangible assets and other assets of the Group's cash-generating units. The Group's investment in its Indian joint venture has also been reviewed for impairment.

Determining whether goodwill or other non-current assets are impaired requires an estimation of the value in use of the business being tested for impairment and of the cash-generating units to which these assets have been allocated. The value in use calculation requires the entity to estimate the future cash flows expected to arise from the cash-generating unit, taking into account the achievability of long-term business plans and macroeconomic assumptions underlying the valuation process, and a suitable discount rate in order to calculate present value. The discount rates used are based on the Group's weighted average cost of capital adjusted to reflect the specific economic environment of the relevant cash-generating unit.

The carrying amount of goodwill at the balance sheet date was £54,712,000 (2016: £54,712,000) and of intangible assets arising from acquisitions was £1,333,000 (2016: £3,953,000). The carrying value of the Group's investment in the Indian joint venture was £4,619,000 (2016: £4,468,000) at the balance sheet date.

Contingent liabilities

On an ongoing basis the Group is a party to various legal disputes, the outcomes of which cannot be assessed with a high degree of certainty. A liability is recognised only where, based on the Group's legal views and advice, it is considered probable that an outflow of resources will be required to settle a present obligation that can be measured reliably. Disclosure of contingent liabilities is made in note 27 unless the possibility of a loss arising is considered remote. These potential liabilities are subject to uncertain future events, may extend over several years and their timing may differ from current assumptions. Management applies its judgement in determining whether or not a liability on the balance sheet should be recognised or a contingent liability should be disclosed.

Retirement benefit obligations

The Group's defined benefit pension scheme has been valued in accordance with IAS 19 'Employee benefits'. The benefit obligation is calculated using a number of assumptions including increases in pension benefits, mortality rates and inflation and the future investment returns from the scheme's assets. The present value of the benefit obligations is calculated by discounting the benefit obligation using market rates on relevant AA corporate bonds at the balance sheet date.

Significant judgement is required in setting the criteria for the valuation of the liability. Effects of changes in the actuarial assumptions underlying the benefit obligation, discount rates and the difference between expected and actual returns on the scheme's assets are classified as actuarial gains and losses.

The defined benefit obligation recognised at the balance sheet date was £21,414,000 (2016: £14,602,000).

Of the items discussed above, revenue and profit recognition represents the key source of estimation uncertainty.