Control is one of the three core elements of the employment status test for IR35. It asks how much the end client directs what the contractor does, and how, when and where they do it. The more the client controls these things, the more employment-like the engagement looks.
A contractor who is told precisely how to carry out the work, who is line-managed, who must keep set hours and who has no autonomy over method is exhibiting strong employment pointers. By contrast, genuine professional autonomy over how the work is done, the ability to organise your own time, and freedom from day-to-day supervision all point towards self-employment and an outside-IR35 position.
Control sits alongside substitution and mutuality of obligation as part of the irreducible minimum from Ready Mixed Concrete [1968] 2 QB 497. Modern authority (PGMOL [2024] UKSC 29) confirms that control is a necessary precondition but is not, on its own, decisive: the whole picture governs. As always, the firm's line is that the actual working practices matter more than the contract wording.