IR35 Compliance, CEST Limitations, and Why a Robust Assessment Process Matters

IR35 continues to be one of the most misunderstood areas of contractor engagement, and engineering and manufacturing businesses are often the ones most exposed. The rules are complex, the consequences of getting them wrong are serious, and the tools available to hiring managers are not always reliable. One of the biggest misconceptions in the market is that HMRC’s CEST tool provides a safe, defensible IR35 determination. The reality is far more nuanced.

The Problem With Relying on CEST

HMRC states that it will stand behind a CEST result only if every answer accurately reflects reality and the engagement operates exactly as described. Even then, this commitment is conditional and does not bind an employment tribunal.

Tribunals have repeatedly overruled CEST outcomes, and HMRC has reserved the right to revisit results where it believes the answers were inaccurate or incomplete.

CEST also has well‑documented limitations:

• It historically ignored key legal tests such as mutuality of obligation, despite the Supreme Court confirming its importance.

• It applies outdated case‑law logic and does not incorporate reasoning from landmark rulings such as Atholl House or PGMOL.

• It provides binary answers with no explanation, making it difficult to defend if challenged.

• HMRC itself acknowledges that CEST has returned incorrect decisions in several cases and that it does not always appeal those outcomes.

  
  

In short, CEST is a useful starting point, but it is not a reliable defence strategy.

Hiring Managers Often Struggle With CEST

CEST’s output is entirely dependent on the answers provided. Many hiring managers openly admit they are unsure about several of the questions, especially around substitution, control, and working practices. This uncertainty means the result may not reflect the true nature of the engagement.

Some recruitment agencies will “help” hiring managers answer the questions. While many reputable firms do this responsibly, others shape answers to achieve the desired outcome, often to make roles easier to fill or to protect profit margins. This should be treated with caution. The ultimate responsibility for determining IR35 status sits with the end client, not the agency.

The Consequences of Getting IR35 Wrong

If HMRC challenges an IR35 determination and finds it incorrect, the consequences can include:

• Backdated tax and National Insurance liabilities

• Interest and potential penalties

• Liability flowing up the supply chain

• Reputational damage

• Disruption to contractor engagement

• Increased scrutiny on future assignments

For engineering and manufacturing businesses that rely heavily on contractors, a single incorrect determination can create significant operational and financial risk.

Our Solution: Independent Assessment and Full Supply‑Chain Protection

At PMB Recruitment, we made a conscious decision not to rely on CEST. Instead, we use an external specialist assessment system that goes far deeper than the government tool. It examines a wider set of questions, covers more detailed working‑practice scenarios, and applies current case law rather than outdated logic.

For assignments that fall into the “grey zone,” specialist finance consultants manually review the engagement and issue a determination. This ensures borderline cases are handled with expert judgement rather than automated logic.

Every contractor is assessed individually against each assignment before commencement. Once the assessment is complete, we apply an IR35 insurance policy that protects the entire supply chain under Article 10. The insurance covers:

• Responding to HMRC IR35 status challenges

• Professional legal defence

• Potential HMRC claims following a challenge

The strength of the system means we never reach the claim stage. The checks, balances, and documented assessment process are clear, consistent, and robust enough to satisfy HMRC.

We fund and manage this insurance ourselves. It means we operate on lower profit margins, but we do it knowing every assignment is watertight and that PMB, the contractor, and the end client are protected.

Compliance comes first. Profit comes second.

A Hard Question for Employers

If you’re engaging contractors or interim specialists on outside IR35 assignments, it’s worth taking a moment to look at the foundations of your compliance process.

Was your determination based purely on the CEST tool?

Did a recruitment agency guide you through the answers?

Are you genuinely confident that your assessment would survive an HMRC challenge?

Most IR35 failures happen quietly. A misunderstood question. An assumption. A “helpful” agency shaping answers. Everything looks fine until HMRC starts asking for documentation, and suddenly the gaps are obvious. That’s when businesses face years of backdated tax, interest, penalties and supply‑chain disruption.

If any part of your process feels uncertain or undocumented, that’s a warning sign.

At PMB Recruitment, we remove that uncertainty. Our independent assessments, specialist reviews and supply‑chain insurance give you a defensible IR35 process that protects your business and your contractors. No guesswork. No loopholes. No sleepless nights.

So ask yourself: are your outside IR35 contractors genuinely compliant, or are you relying on hope that CEST was correct, and a nod from a Recruiter.

If you’re not completely confident, speak with us. We’ll give you clarity and protection before HMRC ever asks the question.