Have to say I totally agree with the following, expressed in a recent post by Thomas Wallace: 'So for example, say that you answered that you had the right to substitute (and incidentally I do not agree with those that claim such a right makes it automatically outside IR35), but when the working practices were properly reviewed by HMRC it is clear that such a clause could almost never be invoked, then the SDS becomes worthless and HMRC will say reasonable care has not been taken.'

Just having lines in a contract DOES still have some influence if well-worded, but insufficient to guarantee an infallible outside IR35 status. However, if, as Thomas infers, the working practices, arrangements and capabilities evidence that right is robust, unfettered and feasible, that's a different story altogether. In 36 appeals cases, HMRC won 17 out of 26 where right of substitution was weak. They won zero from 10 where it was strong. They'll take a risk-based approach when disputing. Sort your right of substitution AND capability/feasibility and they won't come near. Nor should they because your status is legitimate.