The precise origins of due diligence are unclear. The concept of diligence goes back to the Roman law concept of diligentia, distinguishing two main types of diligence: diligentia quam suis rebus, or the care that an ordinary person exercises in managing his or her own affairs, and diligentia exactissima or diligentia boni patrisfamilia, a more exacting type of care exercised by the head of a family.
In The Amstelslot  2 Lloyd’s Rep. 223 per Lord Devlin at 235 :
It is important to get clear the point to which criticism must be directed. There is here no lack of care and no lack of skilled knowledge. The surveyors were quite familiar with the three methods of examination which it is said that they should have adopted; and they could easily have followed them if they had chosen to do so. What is said against them is that by deciding in effect that these methods were not appropriate to the sort of examination they were conducting, they made an error of judgment which a competent surveyor ought not to have made. Lack of due diligence is negligence; and what is in issue in this case is whether there was an error of judgment that amounted to professional negligence.
…He was asked in respect of each of the three suggested precautions whether there was ‘a better chance’ of the crack being found if it had been adopted; and he answered in each case that there was. Some of the other three witnesses gave a similar answer to one or more of the precautions suggested.
My Lords, I fully accept these answers which were the only answers that on the facts of this case could have been made to such a question. After all, there would be a better chance of avoiding road accidents if everyone drove at five miles an hour. This line of questioning is quite legitimate for the purpose of laying a foundation, but it does no more than that. The ultimate question is not whether there was a better chance of discovering the crack, but whether, on balance, one or more of the precautions ought to have been taken. What has to be balanced is in the one scale the extreme unlikelihood of there being any crack to be found and in the other scale the serious damage and loss that could occur if there were a crack. The balance that is so struck will determine whether or not a prudent surveyor should have felt it necessary to do more than these surveyors did.
at pp.230-231 per Lord Reid:
In a great many accidents it is clear after the event that if the defendant had taken certain extra precautions the accident would or might have been avoided. The question always is whether a reasonable man in the shoes of the defendant, with the skill and knowledge which the defendant had or ought to have had, would have taken those extra precautions. …The respondents rely on admissions by the appellants’ witnesses that in making an inspection they ought to look for cracks, and they argue that that must imply that they ought to take every step which would improve their chance of finding a crack. But they do not carry that argument to its logical conclusion. The best chance of finding a crack of this kind is to apply the magnaflux test, but they have been forced to admit that that would not have been a reasonable step to take in this case. So the steps which they say should have been taken would only have been a second best. There must be some compromise or balance in deciding what steps to take in any particular case, keeping in mind both the serious consequences which may flow from failure to detect a defect and the remoteness of the chance that such a defect may exist; for it would plainly be impracticable to make elaborate scientific tests for every defect which could possibly be present in any part of the machinery surveyed.
Per Auld LJ in The Kapitan Sakharov  2 Lloyd’s Rep. 255 at p. 266:
The Judge correctly took as the test whether [shipowner] had shown that it, its servants, agents or independent contractors, had exercised all reasonable skill and care to ensure that the vessel was seaworthy at the commencement of its voyage, namely, reasonably fit to encounter the ordinary incidents of the voyage. He also correctly stated the test to be objective, namely to be measured by the standards of a reasonable shipowner, taking into account international standards and the particular circumstances of the problem in hand.
Papera Traders Co Ltd v Hyundai Merchant Marine Co Ltd (The Eurasian Dream),  All ER (D) 101 (Feb) by Cresswell J at para 130:
The exercise of due diligence is equivalent to the exercise of reasonable care and skill.