The few cases there are stress that the starting point is to review the words in the context of the whole contract using the normal rules of contractual interpretation. Further guidance is then found in the unanimous Court of Appeal decision of West Faulkner Associates v London Borough of Newham [1994] 71 BLR 1. The phrase essentially means:

"to proceed continuously, industriously and efficiently with appropriate physical resources so as to progress the works steadily towards completion substantially in accordance with the contractual requirements as to time, sequence and quality of work".

The Court said it was impossible to give useful guidance beyond this formulation and that, "like the elephant", the failure to proceed regularly and diligently is "far easier to recognise than to describe".

However, a good touchstone by which to judge whether a contractor is proceeding regularly and diligently is to consider the extent to which there is successful progress towards the achievement of contractual obligations.

The Court also clarified that although the two words "regularly" and "diligently" import discrete concepts into the obligation, they should not be considered separately as they partly overlap. This means a contractor must proceed regularly and diligently. However, the Court also said that a contractor can in appropriate circumstances be dismissed from the site if he fails to do either.

Keating on Construction Contracts, one of the leading construction law textbooks, describes the West Faulkner definition as very wide and suggests that "almost any failure by the contractor to comply with a major contractual requirement would amount to a failure to proceed regularly and diligently".

The West Faulkner guidance was recently affirmed and elaborated in two more recent English Technology and Construction Court cases, SABIC UK Petrochemicals Ltd v Punj Lloyd Ltd [2013] EWHC 2916 (TCC) and Vivergo Fuels Ltd v Redhall Engineering Solutions Ltd [2013] EWHC 4030 (TCC).

In SABIC, the contractor was required to carry out and complete the works "with due diligence" in accordance with the contract. The judge confirmed that the due diligence obligation is to be directed towards discharging those contractual obligations which relate to completing the works.

Delay itself is not conclusive proof of a lack of due diligence, but may suggest and evidence a lack of due diligence and call for an explanation.

Neither is the obligation to use due diligence an absolute promise to achieve a particular outcome. Nonetheless, if an outcome is or has become impossible to achieve, it is still relevant when considering whether the separate obligation to achieve due diligence has been met. Put another way, the due diligence obligation does not become less onerous if it is or becomes impossible to achieve a particular contractual object. In such cases, due diligence should be used to minimise any ongoing breach. This may include adopting accelerative measures.

The amount of due diligence will vary throughout the life of a project, depending on the contractual objects at hand. This view was supported unanimously in another Court of Appeal case in 2015, which held that an obligation to proceed with the works with due expedition and without delay "is not directed to every task on the contractor’s to-do list [but] principally…to activities which are or may become critical".

In Vivergo, the judge also approved the West Faulkner definition of the obligation to proceed "regularly and diligently" and gave examples of failures which might indicate a breach of the obligation:

- failure to achieve programmed productivity, as this may demonstrate a lack of resources. At the same time, the judge made it clear that where an employer encourages a contractor to redeploy resources from one area to another (higher priority) area to mitigate alleged delays, the employer will find it harder to argue that the contractor has failed to resource the works adequately, even if the employer's action was caused by the contractor's lack of productivity in the first place;

- failure to supervise workers on site for a sufficient period of the working day is not a separate ground for establishing a failure to proceed regularly and diligently but does provide further support for the case that a contractor is responsible for alleged low productivity;

- failure to produce a proper programme for planning and monitoring the works may in practice prevent a contractor from proceeding continuously, industriously and efficiently, but is not conclusive evidence as, in theory, a contractor without a proper overall programme could proceed regularly and diligently if it deployed proper resources to complete the works on time.

Finally, care is required if the decision is taken to issue a default notice warning a contractor of a failure to proceed regularly and diligently with the works (which is usually required before the issue of a separate termination notice). If a contractor makes urgent and serious attempts to increase productivity to cure failures identified in a default notice before a termination notice is issued (for example, by revising its programme), the employer might not be entitled to terminate and instead find itself facing a claim from the contractor that the employer has repudiated the contract.