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Your client ran CEST. It came back Outside, or Inside, or it shrugged and said Undetermined. Either way, you now have a one-word answer to a question that has occupied the Supreme Court for the best part of a decade.

That gap is worth taking seriously, and the clearest way to see it is through a single case: PGMOL. It is not an IR35 case. It is a dispute about football referees. But it ran from 2018 to May 2026, went through the First-tier Tribunal, the Upper Tribunal, the Court of Appeal, and the Supreme Court, and ended with a tribunal applying exactly the kind of multifactorial reasoning that CEST was never built to do. If you want to understand what a CEST result actually means, this is the case that shows you.

What CEST checks, and what it does not

CEST works through a fixed set of questions on personal service and substitution, control, and financial risk, then returns one of three results: Outside IR35, Inside IR35, or Unable to Determine. There is no score, no weighting, and no written reasoning. We cover the mechanics of that design in detail in our piece on whether CEST is accurate.

The point worth isolating here is narrower. CEST has no test for mutuality of obligation at all. Not a weak one, not a simplified one. It is structurally absent from the questionnaire. A tribunal, by contrast, treats mutuality of obligation as one of the three foundational conditions set out in Ready Mixed Concrete (South East) Ltd v Minister of Pensions (1968): personal service, control, and a wider set of terms consistent with employment. CEST is checking two and a half of those three conditions, at best.

There is a second structural gap. If CEST's substitution questions return a genuine, unfettered right of substitution, the tool can return Outside and stop there, regardless of what the rest of the picture looks like. A tribunal does not work that way. Substitution is one factor weighed against everything else, not an automatic exit. A contract can have a watertight substitution clause and still be found inside IR35 if the rest of the working relationship points firmly toward employment.

The case that shows the gap: PGMOL

Professional Game Match Officials Ltd engages part-time referees to officiate individual football matches. HMRC argued that each match engagement was a contract of employment. PGMOL argued the referees were self-employed. The dispute reached the Supreme Court in 2024.

The Supreme Court's ruling in HMRC v Professional Game Match Officials Ltd [2024] UKSC 29 was widely read as a win for HMRC, and on its narrow legal point, it was. The Court held that mutuality of obligation and sufficient control can exist within a single, short engagement, even where there is no obligation between engagements and either side can cancel without penalty. That is a meaningfully harder standard for contractors than many advisers had previously assumed. If you have been told that having no obligation between assignments keeps you outside IR35, PGMOL narrows that defence. It does not strengthen it.

What the Supreme Court did not do is decide that the referees were employees. It sent the case back to the First-tier Tribunal to weigh the third Ready Mixed Concrete condition: the overall picture, including whether the referees were genuinely in business on their own account. Our explainer on the PGMOL ruling covers that 2024 decision in full.

On 1 May 2026, the First-tier Tribunal delivered that remitted judgment, reported as Professional Game Match Officials Ltd v HMRC [2026] UKFTT 654 (TC). It found that the referees were not employees. Crucially, it did not get there by revisiting mutuality of obligation. The Supreme Court had already settled that mutuality existed in a minimal, technical sense. The tribunal instead looked at the quality of that mutuality and the whole working relationship: the referees could decline appointments freely, withdraw after accepting one without penalty, and most of them treated refereeing as something fitted around full-time work elsewhere rather than a primary income. Taken together, the tribunal found the relationship inconsistent with employment, even though the minimum legal threshold for mutuality had technically been crossed.

That is the gap in one sentence. Mutuality of obligation existed. Sufficient control existed. The referees were still found not to be employees, because a tribunal looks at the overall, qualitative picture, and CEST cannot do that kind of weighing at all.

Why this matters beyond football

PGMOL is not an IR35 case, and the referees were sole traders, not contractors operating through a personal service company. But the reasoning is the same reasoning a tribunal applies to any IR35 dispute, because it uses the same three-part test from Ready Mixed Concrete. Two things carry across directly.

First, the case confirms that satisfying mutuality of obligation and control gets HMRC through the door, nothing more. It does not win the case. The result still turns on the wider picture: how much genuine choice you have, whether the engagement is genuinely episodic, and whether you are operating as a business in your own right. That last point is where HMRC v Atholl House Productions Ltd [2022] EWCA Civ 501 remains the leading authority. The Court of Appeal held that the overall, in-business-on-own-account evaluation has to consider your wider working life, not just the single engagement in front of the tribunal. A portfolio of concurrent clients, genuine financial risk, and real business infrastructure are not decoration. They are the evidence that decides finely balanced cases.

Second, and this is the CEST point, none of that nuance is visible to a tool that returns one word. CEST cannot weigh "mutuality existed but was narrow, short-lived, and driven by choice" against "control existed but was regulatory rather than managerial." A tribunal can, and in PGMOL, that weighing is what decided the case.

What an Outside, Inside, or Undetermined result actually means

If CEST said Outside. Treat it as a starting point, not a guarantee. HMRC has said it will stand behind a CEST result provided the answers given were accurate and in good faith. That protection depends entirely on your answers reflecting how you actually work. If your working practices have drifted from what the contract says, or from what you told CEST, the result is built on a foundation that may not hold. This is the territory the Autoclenz Ltd v Belcher (2011) principle covers: a tribunal will look at the true working arrangement, not just the paper version of it.

If CEST said Inside. This is not necessarily wrong, but it is worth knowing why. Because CEST has no mutuality of obligation test and stops checking once a clean substitution answer is given, an Inside result can sometimes reflect the limits of the questionnaire rather than a genuine employment relationship. It is worth understanding which factors actually drove the result before accepting it as the final word.

If CEST said Undetermined. Roughly one in five CEST assessments end this way, and it is the result most likely to be misunderstood. Undetermined does not mean neutral or safe. It means the tool could not reach a conclusion, and the burden of deciding your status, and defending that decision if HMRC asks, has not gone away. It has simply not been answered.

In every case, the underlying question is the one CEST cannot fully ask: if a tribunal looked at your whole working relationship the way it looked at PGMOL's, what would it find?

See how your contract and working practices weigh up against the same six dimensions a tribunal would consider, with case law reasoning for each, using our contract checker.

The honest summary

CEST is not malicious and it is not simply wrong. It is a fixed questionnaire applied to the answers you give it, built around HMRC's published view of the off-payroll rules, and it omits mutuality of obligation entirely while treating substitution as a possible early exit. A tribunal does neither of those things. It weighs the full picture, as PGMOL shows in detail, and it can reach a different answer from CEST on the same facts, in either direction.

That is the most useful thing to take from PGMOL, even though it is a case about football. The Supreme Court raised the bar on mutuality of obligation. The First-tier Tribunal then showed that clearing that bar settles almost nothing on its own. Whatever CEST has told you, the real question is still the one a tribunal would ask: not whether the boxes are technically ticked, but what your whole working relationship actually looks like.

This article models how a tribunal applying Ready Mixed Concrete, Autoclenz, and PGMOL reasoning would approach the factors CEST checks, for comparison purposes. It is not an official CEST determination and does not constitute legal or tax advice. Always consult a qualified contractor accountant before making decisions about your IR35 status.