What is the test of control of a professional contractor?

In Whittaker v. Minister of Pensions, [1967] 1 Q.B. 156, [1966] 3 All E.R. 531, Mocatta J., with regard to the test of control, said [p. 167]: “It seems clear, therefore, from the more recent cases that persons possessed of a high degree of professional skill and expertise, such as surgeons and civil engineers, may nevertheless be employed as servants under contracts of service, notwithstanding that their employers can, in the nature of things, exercise extremely little, if any, control over the way in which the skill is used. The test of control is, therefore, not as determinative as used to be thought to be the case, though no doubt it is still of value in that the greater the degree of control exercisable by the employer, the more likely it is that the contract is one of service.”