Like I said above:

I guess what I'm trying to say is: don't fret too much about the contractual clause on RoS (but obv. have a good one), and instead try and actually get a sub in, even for a very short time. This would pretty much bullet proof your IR35 defence.

HMRC can try to disprove MOO and SDC, even if you have pretty solid working conditions to support your case. But they cannot argue a RoS that has actually been effected, even if only for a day, and even if the contract does not have an unfettered clause.